The US has ramped up its monitoring of Iranian sleeper cells after president Donald Trump’s strikes on three major Iranian nuclear sites decisively propelled the US into Israel’s war.

Following the strikes, which Mr Trump claimed “totally obliterated” Iran’s nuclear sites of Fordow, Natanz and Isfahan, both White House and FBI officials have been on high alert for Iranian sleeper cells.

Sleeper cells comprised of spies or terrorists hiding out in the US or Western countries, remain inactive, often living quiet and unassuming lives working regular jobs until they are ordered to act on a mission.

(Think Philip and Elizabeth Jennings, the fictional Cold War-era KGB spies who pose as the typical American couple with kids in suburban DC in FX’s series The Americans.)

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A satellite image revealed at least one crater at Iran's Natanz nuclear enrichment site after the US targeted the underground portion of the facility.

A hole of around 5m could be seen in images taken by Maxar Technologies on Sunday after the American strikes.

That hole sits directly over the underground portion of the site, which includes centrifuge halls.

Iran has offered no assessment of how much damage has been done at the site.

North Korea this morning said it strongly condemns the US strike against Iran as a grave violation of a sovereign state's security interests and territorial rights, the North's state media reported.

The US and Israel are the culprits of the current tensions in the Middle East, born out of Jerusalem's "ceaseless war moves and territorial expansion" accepted and encouraged by the West, North Korea's foreign ministry said."

(We) strongly denounce the attack on Iran by the US, which ... violently trampled down the territorial integrity and security interests of a sovereign state."

"The just international community should raise the voice of unanimous censure and rejection against the US and Israel's confrontational acts," the foreign ministry statement said.

Japan has today called for de-escalation of the conflict in Iran and said US strikes demonstrated Washington's determination to prevent Tehran from acquiring nuclear weapons.

Japan also said it was "extremely regrettable" that the situation between Israel and Iran had escalated into a cycle of retaliation, according to foreign minister Takeshi Iwaya."

Japan continues to strongly hope that the path to dialogue will be reopened by efforts toward a resolution of Iran's nuclear issue through talks between the US and Iran," Mr Iwaya said in a statement.

President Donald Trump said the US had "obliterated" Tehran's key nuclear sites, joining Israel in the biggest Western military action against the Islamic Republic since its 1979 revolution.

UN nuclear body chief Rafael Grossi said damage at Iran's main uranium enrichment plant at Fordo following a US airstrike can't yet be assessed.

Mr Grossi told an emergency meeting of the UN Security Council called by Iran that craters are visible at the Fordo site, indicating the US use of ground penetrating munitions, but the underground damage cannot be assessed yet.

At the Isfahan site, the head of the International Atomic Energy Agency said additional buildings were hit, some related to converting uranium, and "entrances to tunnels used for the storage of enriched material appear to have been hit".

At the Natanz nuclear enrichment site, Mr Grossi said the fuel enrichment plant has been hit again.

"At this time, no one, including the IAEA, is in a position to assess the underground damage at Fordo," Mr Grossi said.

The US state department has doubled the number of emergency evacuation flights it is providing for American citizens wishing to leave Israel.

It also ordered the departure of nonessential staff from the US embassy in Lebanon and is stepping up travel warnings around the Middle East amid concerns Iran will retaliate against US interests in the region.

In internal and public notices, the department over the weekend significantly ramped up its cautionary advice to Americans in the Middle East.

In a notice yesterday, after American strikes against Iran's nuclear facilities, the department said it had ordered nonessential personnel and the families of staff at the US embassy in Beirut to leave Lebanon "due to the volatile and unpredictable security situation in the region".

Israel was very close to meeting its goals in Iran of removing the threats of ballistic missiles and the nuclear programme, prime minister Benjamin Netanyahu said on Sunday.

He said: "We won’t pursue our actions beyond what is needed to achieve them, but we also won't finish too soon. When the objectives are achieved, then the operation is complete and the fighting will stop."

"I have no doubt that this is a regime that wants to wipe us out, and that's why we embarked on this operation to eliminate the two concrete threats to our existence: the nuclear threat, the ballistic missile threat.

“We are moving step by step towards achieving these goals. We are very, very close to completing them.”

Australia has voiced its support for the U.S. strike on Iran, while simultaneously urging for de-escalation and a return to diplomatic efforts.

Foreign Minister Penny Wong confirmed Canberra's stance on Monday, stating in a television interview with Seven Sunrise: “We support action that the U.S. has taken to prevent Iran from obtaining a nuclear weapon.”

Speaking across a series of interviews, Ms Wong characterised the strike as a unilateral action by a key security ally. She emphasised that Australia is joining Britain and other nations in calling for Iran to re-engage in negotiations.

Reiterating the urgent need to avoid further conflict, Ms Wong told reporters in Canberra: “We do not want to see escalation.”