Iran's supreme leader Ayatollah Ali Khamenei on Monday praised Hamas's unprecedented October 7 attack against Israel and predicted the "destruction" of their common enemy.

Khamenei, 85, was speaking at an event to mark 35 years since the death of Ayatollah Ruhollah Khomeini, the founder of the Islamic Republic which replaced a US-backed monarchy.

He said the October 7 attack by Palestinian Islamist militant group Hamas "was a decisive blow to the Zionist regime" and put Israel "on the path that will only end in its destruction".

Since the 1979 Islamic revolution, Iran, the main Shiite Muslim power, has emerged as the bitter enemy of Israel and that country's Western allies the United States and Britain.

Iran is under international sanctions over its contested nuclear programme which it insists is for civilian purposes.

While Israel and Iran have long fought a shadow war of killings and sabotage, Iran's armed allies across the Middle East have formed a so-called "Axis of Resistance" alliance.

As the Gaza war has raged, Iran and Israel came to the brink of war in mid-April when Tehran launched a barrage of rockets and missiles at Israel, most of which were intercepted.

Iran has said it had no advance knowledged of Hamas's October 7 attack but has praised it since.

Khamenei said the attack "happened at the right time" and "destroyed a major international conspiracy for the Middle East," a possible reference to US-led moves to broker diplomatic ties between Israel and Arab powers.

The Gaza war was sparked by Hamas's October 7 attack, which resulted in the deaths of 1,190 people, mostly civilians, according to an AFP tally based on Israeli official figures.

Militants also took about 250 hostages, 120 of whom remain in Gaza, including 37 the army says are dead.

Israel's retaliatory bombardments and ground offensive have killed at least 36,439 people in Gaza, mostly civilians, the Hamas-run territory's health ministry said on Sunday.

Khamenei, who spoke to thousands gathered in the Khomeini mausoleum near Tehran, also said that "the Zionist regime is gradually melting before the eyes of the people of the world".

"Sooner or later, America will have to withdraw its support," he added.