Iranian President Hassan Rouhani said on Saturday that "terrorists" were applauding his US counterpart Donald Trump for launching a missile strike on an airbase of his Syrian government ally.

But he backed calls for an independent inquiry into a suspected chemical weapons attack on a rebel-held town in northwestern Syria on Tuesday that Trump blamed on the Damascus regime.

"This man who is now in office in America claimed that he wanted to fight terrorism but today all terrorists in Syria are celebrating the US attack," Rouhani said in a speech aired by state television.

"Why have you attacked the Syrian army which is at war with terrorists? Under what law or authority did you launch your missiles at this independent country?"

Iran and Russia are the closest allies of Syrian President Bashar al-Assad.

They have provided him with military support not only against jihadists like the Islamic State group and former Al-Qaeda affiliate Fateh al-Sham Front that are targeted by a US-led coalition but also against other rebels they deem "terrorists" too.

Satellite image released of Syrian air-strike target

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Both governments have defended their Damascus ally against Western allegations that it carried out a chemical weapons attack on the rebel-held town of Khan Sheikhun on Tuesday, killing dozens of civilians.

Rouhani called for "an independent commission" by "impartial countries" into the claims.

"According to the United Nations, the Syrian government does not possess chemical weapons," he said.

Rouhani was referring to the UN-supervised destruction of the Damascus regime's chemical arsenal under a 2013 agreement between Washington and Moscow.

On Friday, hours after the US missile strike, Iranian Foreign Minister Mohammad Javad Zarif said the Western allegations were "bogus".

Syrian army spokesman on US attack

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He likened them to the claims that Saddam Hussein's regime had weapons of mass destruction which premised the US-led invasion of Iraq in 2003 but which later turned out to be baseless.

It comes after US Treasury Secretary Steve Mnuchin told reporters that he plans to announce additional economic sanctions aimed at Syria in the near future.

"We expect that those (sanctions) will continue to have an important effect on preventing people from doing business with them," Mnuchin said on Friday.

"These sanctions are very important and we will use them the maximum effect."

US Ambassador Nikki Haley has delivered a warning at an emergency session of the UN Security Council.

"The United States took a very measured step last night," Haley told the council.

"We are prepared to do more, but we hope it will not be necessary."

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Haley said the strike destroyed an air field from which Washington believes Damascus launched the attack on rebel-held Khan Sheikhun, where 86 people including 27 children died this week.

"We were fully justified in doing so," she said.

Russia accuses US

The United States did not seek Security Council authorisation for the military action that followed days of global outrage at images of dead children from the suspected sarin gas attack.

It was Trump's biggest military decision since taking office and marked a dramatic escalation in American involvement in Syria's protracted war.

"The United States attacked the territory of sovereign Syria," Russian Deputy Ambassador Vladimir Safronkov told the council, denouncing a "flagrant violation of international law and an act of aggression."