Negotiating teams from the U.S. and Iran could return to Islamabad ​this week to resume talks to end the war, sources told Reuters on Tuesday, after the collapse of weekend negotiations prompted Washington to impose a blockade on Iranian ports.

While the ‌U.S. blockade drew angry rhetoric from Tehran, signs that diplomatic engagement might continue helped calm oil markets, pushing benchmark prices below $100 on Tuesday.

The highest-level talks between the two adversaries since the 1979 Islamic Revolution ended in the Pakistani capital without a breakthrough at the weekend, raising doubts over the survival of a two-week ceasefire that still has a week to run.

But a source involved in the talks said on Tuesday both countries could return as early as the end of this week, and that a proposal had been shared ​with Washington and Tehran to resend their delegations.

“No firm date has been set, with the delegations keeping Friday through Sunday open,” a senior Iranian source said.

U.S. President Donald Trump said Iran had ​been in touch on Monday and wanted to make a deal, adding that he would not sanction any agreement that allowed Tehran to possess a nuclear weapon.

Since ⁠the United States and Israel began the war on February 28, Iran effectively shut the Strait of Hormuz to nearly all vessels except its own, saying passage would be permitted only under Iranian control and ​subject to a fee. The fallout has been widespread, as nearly a fifth of the world’s oil and gas previously flowed through the narrow waterway.

In a countermeasure, the U.S. military said it began blocking shipping traffic in ​and out of Iran’s ports on Monday.

Tehran has threatened to hit naval ships going through the strait and to retaliate against its Gulf neighbours’ ports.

Nearly 24 hours into the U.S. blockade, there had yet to be reports of Washington taking direct action against shipping to enforce it.

Three Iran-linked tankers were seen transiting the Strait of Hormuz on Tuesday, shipping data showed, but the vessels were not heading to or from Iranian ports.