Brent crude, the international standard, fell 3.8% to $73.87 a barrel. It has been trading below $80 in recent days but is still above the roughly $70 per barrel it was trading at in late February before the war began.

U.S. crude prices fell 3.9% to $70.34 a barrel. Early Thursday, Brent was down 1.3% at $72.90, while U.S. benchmark crude lost 1.4% to $69.37.

The drop in prices comes as shipping across the Strait of Hormuz slowly resumed last week under the U.S.-Iran memorandum of understanding.

Secretary of State Marco Rubio was in Bahrain Thursday after promising Gulf allies that Washington would protect their interests as it seeks to hammer out a final settlement to end the war with Iran.

During a visit to Kuwait City, Rubio said Washington would be on the same page as Gulf states as it wrangles with Iran over a permanent settlement to the conflict.

"We're going to be completely aligned with our partners in the Gulf," he said, adding that the United States would "engage them on conversations about every decision that's made with regards to this negotiation."

Rubio is due to attend a Gulf Cooperation Council meeting in Bahrain on Thursday after sitting down with the leaders of Kuwait and the United Arab Emirates on Wednesday.

The initial US-Iran deal, which sets out a 60-day negotiating process aimed at reaching a long-term agreement, did not to address Gulf nations' long-standing concerns about Iran's missile program.

But Rubio insisted Washington was "not going to do anything that undermines the security of our allies".

The Senate late Wednesday rejected a measure aimed at restricting President Trump's power to wage war against Iran, in a victory for the president and Senate GOP leadership as they seek to quell congressional discontent with the Trump administration's Iran strategy.

The procedural motion failed in a 50 to 47 vote, with two Republicans — Sens. Susan Collins and Lisa Murkowski — voting in favor of advancing the resolution by Democratic Sen. Tim Kaine, siding with most Democrats. Republican Sen. Rand Paul voted present, and Democratic Sen. John Fetterman voted no.

Just one day earlier, four Republicans voted yes on a separate House-passed resolution to restrict Mr. Trump's war powers, allowing it to narrowly pass. Those same four GOP lawmakers had voted to advance the Kaine resolution in an earlier procedural vote last month — the first time an Iran war powers resolution had moved forward in the Senate after seven failed attempts.

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The Israeli military said Thursday that a soldier was killed the previous day in southern Lebanon, where clashes with Iran-backed Hezbollah have been reported despite a ceasefire.

Master Sergeant Basil Sweid, 32, a driver, "fell during operational activity," the military said.

A spokesman told AFP he was killed when his vehicle overturned.

The military says 37 soldiers and one civilian contractor have been killed in southern Lebanon since fighting with Hezbollah erupted in early March.