It follows the start of military exercises by Iran's Revolutionary Guards on Friday near the Iraqi border in the western region of Kermanshah.

Following a conversation between US Defence Secretary Lloyd Austin and his Israeli counterpart on Sunday, the Pentagon verified the sub's deployment and expedited the Abraham Lincoln carrier strike group's arrival in the area.

"Secretary Austin reiterated the United States' commitment to take every possible step to defend Israel and noted the strengthening of US military force posture and capabilities throughout the Middle East in light of escalating regional tensions," stated a statement.

Earlier, an Iranian armed forces official told the country's official news agency IRNA that its drills were aimed at "enhancing combat readiness and vigilance".

Iran has claimed Hamas's top political leader Ismail Haniyeh was "assassinated" in its capital Tehran on Wednesday 31 July. Retaliation has been expected since.

Israel has not claimed or denied responsibility for Mr Haniyeh's death.

Western countries have been urging Iran to show restraint - with fears a response could lead to a regional war.

However, sources have told US media the Israeli intelligence community believes Iran has decided to launch a direct attack and will do so within days.

Barak Ravid, a reporter with US media site Axios, said his two sources told him this marks a change in Israel's assessment of Iran's position.

Mr Ravid said one of the sources told him there has been an internal debate in Iran between the Revolutionary Guards and President Masoud Pezeshkian over a potential response to the death of Haniyeh.

The Revolutionary Guards have been pushing for a severe response while Mr Pezeshkian believes this should be avoided, Mr Ravid claims.

The reports come as Israel's war on Gaza is into its tenth month.

The war began when Hamas-led militants burst through Israel's defences on 7 October, killing some 1,200 people - mostly civilians - and abducting around 250 people.

Gaza's Hamas-run health ministry says the number of Palestinians killed in the Israeli response is approaching 40,000.

On Saturday, an Israeli strike on a school-turned-shelter left 80 people dead in the territory, according to Palestinian health officials.

An estimated 6,000 displaced people were sheltering in Tabeen school when it was struck by three missiles, according to a spokesperson for the Civil Defence agency, which operates under the Hamas-run government.