President Joe Biden on Friday warned Iran that the United States would “act forcefully” to protect Americans, after the U.S. military carried out air strikes against Iran-backed forces in retaliation for an attack in Syria.

Later, officials said that another U.S. service member was wounded on Friday in the latest tit-for-tat strike between Iran-backed forces and U.S. personnel in Syria, Reuters reported.

That comes on top of seven casualties on Thursday, which Washington blamed on a drone of Iranian origin, and included an American contractor being killed and five U.S. troops and another contractor being wounded.

Suspected U.S. rocket fire on Friday targeted new areas in eastern Syria, according to two local sources, with no casualties reported. Pro-Iranian forces in Syria said in an online statement late Friday that they have a “long arm” to respond to further U.S. strikes on their positions.

The violence could further aggravate already strained relations between Washington and Tehran, as attempts to revive a 2015 nuclear deal between Iran and major powers stalled, and Iranian drones being used by Russia against Ukraine.

Although U.S. forces stationed in Syria have been attacked with drones before, deaths are rare.

“Make no mistake: the United States does not … seek conflict with Iran, but be prepared for us to act forcefully to protect our people,” Biden told reporters during a visit to Canada.

Asked whether there should be a higher cost for Iran, Biden replied: “We’re not going to stop.”

The Pentagon had said U.S. F-15 jets on Thursday attacked two facilities used by groups affiliated with Iran’s Islamic Revolutionary Guards Corps (IRGC).

The Syrian Observatory for Human Rights, which monitors the war in Syria, said the U.S. strikes had killed eight pro-Iranian fighters. Reuters was unable to independently confirm the toll.

Iran’s state Press TV said no Iranians had been killed and quoted local sources as saying the target was not an Iran-aligned military post, but that a rural development center and a grain center near a military airport had been hit.

The U.S. strikes were a response to a drone attack earlier on Thursday on a base near Hasakah in northeast Syria operated by a U.S.-led coalition battling the remnants of Islamic State.

Three service members and a contractor required medical evacuation to Iraq, while two wounded American troops were treated at the base. On Friday, the Pentagon said the injured personnel were in stable condition.

Two U.S. officials, speaking on condition of anonymity, said it appeared that the defensive system on the base had failed.

The Pentagon said the U.S. military had a complete site picture in terms of radar, though one official told Reuters troops on the ground did not appear to have had enough time to react to the drone.

A U.S. base at the Al-Omar oil field in Syria was attacked on Friday morning, according to the Lebanese pro-Iranian TV channel Al Mayadeen and a security source.

It is not uncommon for Iranian-backed groups to fire missiles at U.S. bases in Syria after they are hit with air strikes.

U.S. forces first deployed into Syria during the Obama administration’s campaign against Islamic State, partnering with a Kurdish-led group called the Syrian Democratic Forces. There are about 900 U.S. troops in Syria, most of them in the east.

U.S. troops have been attacked by Iranian-backed groups about 78 times since the beginning of 2021, according to the U.S. military.

While Islamic State has lost the areas of Syria and Iraq it ruled over in 2014, sleeper cells still carry out hit-and-run attacks in desolate areas where neither the U.S.-led coalition nor the Syrian army exert full control.

North Korea has tested a new nuclear-capable underwater attack drone, state media reported on Friday, as leader Kim Jong Un warned joint military drills by South Korea and the U.S. should stop, Reuters reported.

During the test, the new North Korean drone cruised underwater at a depth of 80 to 150 metres (260-500 feet) for over 59 hours and detonated a non-nuclear payload in waters off its east coast on Thursday, North Korean state news agency KCNA said.

Analysts say North Korea is showing off its increasingly diverse nuclear threats to Washington and Seoul, though they are sceptical whether the underwater vehicle is ready for deployment.

North Korea intends to signal “to the United States and South Korea that in a war, the potential vectors of nuclear weapons delivery that the allies would have to worry about and target would be vast,” said Ankit Panda, senior fellow at the U.S.-based Carnegie Endowment for International Peace.

“There would be silos, railcars, submarines and road mobile missile launchers. And now they’re adding this underwater torpedo to the mix,” he said.

According to Reuters on Monday, the isolated country flew a short-range missile from a buried silo, a departure from usual basing methods.

Dubbed “Haeil”, or tsunami, the new drone system is intended to make sneak attacks in enemy waters and destroy naval strike groups and major operational ports by creating a large radioactive wave through an underwater explosion, the KCNA said.

“This nuclear underwater attack drone can be deployed at any coast and port or towed by a surface ship for operation,” the news agency said, adding that Kim oversaw the test.

A South Korean military official said they were analysing North Korea’s claims. A U.S. official, speaking on the condition of anonymity, said there was no indication of a nuclear test.

It is unclear whether North Korea has fully developed miniaturised nuclear warheads needed to fit on its smaller weapons.

Analysts say perfecting such warheads would most likely be a key goal if the North resumes nuclear testing, read the report.

A photo released by state media showed Kim smiling next to a large torpedo-shaped object, but did not identify it as the new drone. Other photos showed tracks of the object’s underwater trajectory, and blasts visible on the sea surface.

Panda said the weapon’s operational concept was similar to Russia’s Poseidon nuclear torpedoes, a new category of retaliatory weapon meant to create destructive, radioactive blasts in coastal areas.

On Friday South Korean President Yoon Suk Yeol said he would make sure North Korea paid for its “reckless provocations”, during a speech to commemorate service members who died in clashes with North Korea in western waters, including a 2010 sinking of a navy ship that South Korea said was struck by a North Korean torpedo.

North Korea also said it had fired cruise missiles on Wednesday to practice carrying out tactical nuclear attacks, confirming earlier reports from the South Korean military, Reuters reported.

The cruise missiles were tipped with a “test warhead simulating a nuclear warhead,” and flew 1,500-1,800 km (930-1,120 miles), according to KCNA.

The latest tests took place as South Korean and U.S. troops launched their largest amphibious landing drills in years, involving a U.S. amphibious assault ship, on Monday.

North Korea said military exercises by the United States and South Korea require its forces to “gird themselves for an all-out war and bolster up its nuclear force both in quality and quantity on a priority basis”.

Pyongyang has long bristled at exercises conducted by South Korean and U.S. forces, saying they are preparation for an invasion of the North.

South Korea and the U.S. say the exercises are purely defensive and have criticised the North’s tests as destabilising and in breach of U.N. sanctions.

The allies concluded 11 days of their regular springtime exercises, called Freedom Shield 23, on Thursday, but have other field training exercises continuing.

North Korean leader Kim expressed “his will to make the U.S. imperialists and the South Korean puppet regime plunge into despair for their choice,” KCNA said, adding that he warned the enemies that they should stop reckless anti-North Korea war drills.

The director of the U.S. Defense Intelligence Agency said on Wednesday North Korean leader Kim does not appear poised to carry out a nuclear test during U.S.-South Korea military exercises, but the United States is staying vigilant.

China’s military said on Thursday it had monitored and driven away a U.S. destroyer that illegally entered waters around the Paracel Islands in the South China Sea, Reuters reported.

In a statement, the military said that the guided-missile destroyer USS Milius intruded into China’s territorial waters, undermining peace and stability in the busy waterway.

“The theater forces will maintain a high state of alert at all times and take all necessary measures to resolutely safeguard national sovereignty and security and peace and stability in the South China Sea,” said Tian Junli, a spokesman for China’s Southern Theatre Command.

The U.S. Navy on Thursday disputed the Chinese military statement, saying the destroyer is conducting “routine operations” in the South China Sea and was not expelled.

“The United States will continue to fly, sail, and operate wherever international law allows,” a statement from the U.S. Navy 7th Fleet said.

Tension between the United States and China has been growing in the area.

The United States has been shoring up alliances in the Asia-Pacific seeking to counter China’s assertiveness in the South China Sea and the Taiwan Strait, as Beijing seeks to advance its territorial claims.