This is a locator map for the Gulf Cooperation Council member states: Saudi Arabia, Bahrain, Qatar, Oman, Kuwait and United Arab Emirates. AP hide caption

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DUBAI, United Arab Emirates — A cargo ship caught fire Sunday after being hit by an unknown projectile off Qatar's coast, the British military said, in the latest attack on vessels in the Persian Gulf since a shaky ceasefire stopped fighting between the U.S. and Iran.

The Trump administration says the month-old ceasefire remains in effect. But it has been repeatedly tested with Iran restricting traffic through the Strait of Hormuz, a strategic waterway key to the global flow of oil, and the U.S. imposing a blockade of Iranian ports.

Washington has been awaiting Iran's response to a new proposal for a deal to end the war, reopen the strait to shipping and roll back Iran's nuclear program.

One of the main sticking points in the negotiations is the fate of Iran's stockpile of highly enriched uranium. The U.N. nuclear agency says Iran has more than 440 kilograms (970 pounds) of uranium that is enriched up to 60% purity, a short, technical step from weapons-grade levels.

In an interview with Iranian state media, a spokesman for the Iranian military said that forces were on "full readiness" to protect nuclear sites where the uranium is stored.

"We considered it possible that they might intend to steal it through infiltration operations or heliborne operations," Brig. Gen. Akrami Nia told the IRNA news agency late Saturday. He didn't offer further details.

The majority of Iran's highly enriched uranium is likely still at its Isfahan nuclear complex, the head of the U.N. nuclear agency told The Associated Press last month.

The Isfahan facility was bombarded by U.S.-Israeli airstrikes in the 12-day war last year, and faced less intense attacks in this year's war.

In Sunday's naval attack, the United Kingdom Maritime Trade Operations Centre said that the strike caused a small fire on the ship, which was extinguished. The attack happened 23 nautical miles (43 kilometers) northeast of Qatar's capital, Doha, the UKMTO said.

There were no reported casualties, it said. It gave no details on the owner or origin of the ship, and there was no claim of responsibility.

But there have been several attacks against ships in the Persian Gulf over the past week. On Friday, the U.S. struck two Iranian oil tankers after it said that the vessels were trying to breach its blockade of Iran's ports.

Iran's Revolutionary Guard navy on Sunday reiterated its warning that any attack on Iranian oil tankers or commercial vessels would be met with a "heavy assault" on one of the U.S. bases in the region and enemy ships.

U.S. President Donald Trump has reiterated threats to resume full-scale bombing, if Iran doesn't accept an agreement to reopen the strait and roll back its nuclear program. Iran has mostly blocked the critical waterway for global energy since joint strikes on Feb. 28 by the U.S. and Israel launched the war, which has caused a global spike in fuel prices and rattling world markets.