WASHINGTON -- The US military struck missile launch sites and mine-laying boats in southern Iran on Monday, the US Central Command said.

"US forces conducted self-defense strikes in southern Iran today to protect our troops from threats posed by Iranian forces," Tim Hawkins, spokesperson for the command, said in a statement.

"Targets included missile launch sites and Iranian boats attempting to emplace mines. US Central Command continues to defend our forces while using restraint during the ongoing ceasefire," he said.

The strikes were carried out in the Bandar Abbas area which hosts Iran's main naval base, destroying two vessels of Iran's Islamic Revolution Guard Corps and hitting a surface-to-air missile site, said Hawkins.

Four people were killed in the attack, while the total number of casualties remained unclear, Fars News Agency reported, citing Iranian state television.

Fox News reported that two Iranian boats were spotted laying mines in the Strait of Hormuz, and a missile site targeted US warplanes. In response, US forces conducted "self-defense" strikes.

The US strikes were "over for now," said the report, citing a senior US official on Monday.

The strikes do not indicate the weeks-long US ceasefire with Iran is over, two sources were quoted as saying.

US President Donald Trump said on Monday that negotiations with Iran were "proceeding nicely," but warned that Washington would resume military strikes if the talks failed, with any future attacks being "bigger and stronger than ever before."

Trump also said Iran's enriched uranium would be destroyed after being handed over to the United States, in place, or at another "acceptable location."

Meanwhile, Iran's semi-official Tasnim news agency reported that Tehran had not agreed to transfer enriched uranium abroad, rejecting a report that Iran was "prepared to remove its highly enriched uranium from its territory."

A possible interim deal between the two sides would not include a nuclear agreement, The Washington Post reported on Monday, citing an Iranian official.