"It is still early to assess the results of the operation," Israeli military spokesman Brigadier General Effie Defrin said in a televised press conference, a day after a ceasefire ended 12 days of war between the longtime foes.

"I believe we have delivered a significant hit to the nuclear programme, and I can also say that we have delayed it by several years," he said.

The head of Israel's military, Eyal Zamir, also said late Tuesday that Israel and the US had set back Iran's nuclear programme "by years".

But a classified preliminary US intelligence report concluded that the American strikes with bunker-busting bombs on Iranian nuclear sites at the weekend had delayed the programme by just a few months.

US media on Tuesday cited people familiar with the Defense Intelligence Agency findings as saying the American strikes did not fully eliminate Iran's centrifuges or stockpile of enriched uranium.

The strikes sealed off entrances to some facilities without destroying underground buildings, according to the report.

Israel had said its bombing campaign, which began on June 13, was aimed at preventing Iran from acquiring a nuclear weapon, an ambition Tehran has consistently denied.

Israeli Prime Minister Benjamin Netanyahu, in an address to the nation after the ceasefire, announced that "we have thwarted Iran's nuclear project".

US President Donald Trump claimed after the US strikes that the Iranian programme had been "totally obliterated."