"Americans know now that their bases can be targeted by Iran ... Their bases will be targeted if the United States responds to Iran's missile attacks in Iraq," state TV cited the Guards as saying on Wednesday.

A senior official in Supreme Leader Ayatollah Ali Khamenei's office said Iran's response to the killing of its top commander Qassem Soleimani so far had been the "weakest" of the Iranian revenge scenarios, state TV reported.

Iran struck back at the US for the killing of a top Iranian general, firing a series of surface-to-surface missiles at an Iraqi air base housing US troops and warning the United States and its allies in the region not to retaliate.

US officials confirmed the air strikes at the air base and the Pentagon said Iran launched over a dozen ballistic missiles targeting at least two military bases in Iraq where US and coalition troops were based.

US Defence Department spokesman Jonathan Hoffman said the attacks "targeted at least two Iraqi military bases" at Ain Assad and Irbil.

Australian troops are located close to where the attacks took place, but Prime Minister Scott Morrison said all Australian military personnel and staff in Iraq were safe.

"We're doing everything to ensure their continued safety," Scott Morrison said.

The Prime Minister said the situation in Iraq was "fluid".

A US official says there are very few, if any, casualties from the missile attack. The official spoke on condition of anonymity in advance of a Pentagon briefing.

The official says 15 missiles were fired. Ten struck the Ain al-Asad air base in Iraq's western Anbar province. One struck a base in Irbil in Iraq's semi-autonomous Kurdish region. Four missiles failed to hit their targets.

The official says the bases are still being searched for casualties.

Iran's Revolutionary Guard said the Ain Assad airbase was "completely destroyed".

The attack was "a total success by all accounts," the Revolutionary Guard said.

Iranian state TV said the attack was in revenge for the killing of Revolutionary Guard General Qassem Soleimani, whose funeral on Tuesday in his hometown of Kerman prompted angry calls to avenge his death, which drastically raised tensions in the Middle East.

Iran's Revolutionary Guard warned the US and its regional allies against retaliating over the missile attack against the Ain Assad air base in Iraq's western Anbar province. The Guard issued the warning via a statement carried by Iran's state-run IRNA news agency.

"We are warning all American allies, who gave their bases to its terrorist army, that any territory that is the starting point of aggressive acts against Iran will be targeted," The Guard said. It also threatened Israel.

Iranian Foreign Minister Mohammad Javad Zarif tweeted Iran was not seeking escalation or war, but it would defend itself against any aggression.

"Iran took & concluded proportionate measures in self-defence under Article 51 of UN Charter targeting base from which cowardly armed attack against our citizens & senior officials were launched. We do not seek escalation or war, but will defend ourselves against any aggression," he tweeted.

US President Donald Trump says "all is well" following the retaliatory missile attack.

"All is well! Missiles launched from Iran at two military bases located in Iraq. Assessment of casualties & damages taking place now. So far, so good!" Trump wrote in a post on Twitter.

"I will be making a statement tomorrow morning," he added.

Ain Assad air base is in Iraq's western Anbar province. It was first used by American forces after the 2003 US-led invasion that toppled dictator Saddam Hussein. It later saw American troops stationed there amid the fight against the Islamic State group in Iraq and Syria.

State TV said the operation's name was "Martyr Soleimani." It said the Guard's aerospace division that controls Iran's missile program launched the attack. Iran said it would release more information later.

Wednesday's revenge attack came a mere few hours after crowds in Iran mourned Soleimani and as the US continued to reinforce its own positions in the region and warned of an unspecified threat to shipping from Iran in the region's waterways, crucial routes for global energy supplies.

US embassies and consulates from Asia to Africa and Europe issued security alerts for Americans.

A stampede broke out on Tuesday at Soleimani's funeral in Iran and at least 56 people were killed and more than 200 were injured as thousands thronged the procession, Iranian news reports said.

The US blames Soleimani for killing US troops in Iraq and accused him of plotting new attacks just before he was killed.

In Iraq, pro-Iranian factions in parliament have pushed to oust American troops from Iraqi soil following Soleimani's killing.

Mr Morrison has been briefed on the attacks and said the government was monitoring the unfolding events in Iraq.

The Prime Minister spoke to defence chief Angus Campbell and minister Linda Reynolds about the reported air strike on Wednesday.

Mr Morrison said he had directed the Chief of the Defence Force to take whatever actions were necessary to protect and defend our ADF and diplomatic personnel and keep Australians safe.

"The National Security Committee of the Cabinet has been meeting to review this situation and take decisions as necessary since Saturday, is already scheduled to meet again for this purpose on Thursday and will be convened sooner should further information be confirmed requiring such a response, and will be done so in consultation with the CDF."

The Prime Minister learnt about the attack while flying from Canberra to South Australia to visit bushfire-ravaged Kangaroo Island.

Mr Morrison has also spoken with Opposition Leader Anthony Albanese to update him on the unfolding situation.

Mr Albanese has urged the United States and Iran to exercise restraint.

"I don't want to see Australia drawn into a military conflict in the Middle East," he told reporters in Adelaide.

"The first priority should be ensuring that Australians are kept safe."

NRMA spokesman Peter Khoury is concerned about the potential impact on petrol prices.

"The next 24 hours will be critical," Mr Khoury told AAP.

"The US markets are worrying - prices jumped almost immediately by four per cent - if that's a n indication of what's to come, it's very concerning."

America's Federal Aviation Administration is barring US pilots and carriers from flying in areas of Iraqi, Iranian and some Persian Gulf airspace.

The agency is warning of the "potential for miscalculation or mis-identification" for civilian aircraft amid heightened tensions between the US and Iran.

The emergency flight restrictions follow ballistic missile strikes on two Iraqi bases that house US troops.

Such restrictions are often precautionary in nature to prevent civilian aircraft from being confused for ones engaged in armed conflict.

The FAA said the restrictions were being issued due to "heightened military activities and increased political tensions in the Middle East, which present an inadvertent risk to US civil aviation operations."

More to come