US forces attacked missile sites in southern Iran and boats trying to lay mines on Monday, US Central Command said, imperiling a fragile ceasefire and casting new doubt on a deal to end the Middle East war.

The strikes came as top Iranian negotiators arrived in Doha for the latest round of talks to end the months-long conflict, and as the Israeli military stepped up hostilities with Iran-backed Hezbollah in southern Lebanon.

Oil prices fluctuated in the wake of the US strikes, which may threaten any agreement to reopen the Strait of Hormuz, where an Iranian blockade has choked global fuel supplies.

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

It gave no details of the attacks and said only that the targets included missile launch sites and boats trying to "emplace mines."

Iran's state-run broadcaster IRIB reported several loud explosions were heard in the vicinity of Bandar Abbas at around midnight local time (2030 GMT Monday).

It added the situation in the southern port city was normal and local authorities were investigating the cause of the blasts.