Washington DC - President Donald Trump said Friday that only Iran's "unconditional surrender" would bring an end to the joint US-Israeli war that has thrown the entire region into turmoil.

Now in its seventh day, the war has engulfed multiple nations, upended the world's energy and transport sectors, and brought chaos to even usually peaceful areas around the Gulf.

It has spread to Lebanon, whose prime minister warned of an impending humanitarian disaster as tens of thousands fled heavy Israeli strikes on Beirut's southern suburbs.

Trump, who has given varying reasons for starting the war that killed Iran's supreme leader Ayatollah Ali Khamenei last weekend, promised to help rebuild the country he is attacking if Tehran installed an "acceptable" new leader.

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"There will be no deal with Iran except UNCONDITIONAL SURRENDER," Trump wrote on his Truth Social platform.

"MAKE IRAN GREAT AGAIN (MIGA!)," he added.

In Tehran, crowds of men and women dressed in black, some carrying Iranian flags, gathered for the first Friday prayers since the start of the war, online footage showed.

Several loud explosions sent clouds of black smoke into Tehran's sky, according to AFP journalists who described the day's strikes as the heaviest yet on the capital.

Both Israel and the US warned on Friday they were escalating their attacks on Iran.

"We have additional surprises ahead which I do not intend to disclose," Israel's military chief Lieutenant General Eyal Zamir said.

According to Iran's health ministry, the US and Israeli strikes on the country have killed 926 people. Iranian government spokesperson Fatemeh Mohajerani said on Friday that 30% of the dead were children.

Iran has launched missile and drone attacks at Israel and Gulf states since the war began, with AFP journalists in Tel Aviv reporting hearing several blasts on Friday.

In Israel, at least 10 people have been killed, according to first responders there.

The US military has reported the deaths of six of its personnel.