Iran's foreign minister slammed potential US-led airstrikes against Syria as "illegal" on a visit to Baghdad Sunday, while his Iraqi counterpart warned they would hinder efforts towards a political solution.

Mohammad Javed Zarif said military action was barred under the United Nations charter, but Washington is pressing for the strikes in response to an alleged chemical weapons attack the White House says was carried out by the regime of Syrian President Bashar al-Assad.

"Civilised countries, 65 years ago, took the options off the table when they rejected in the charter of the United Nations resort to force as an illegal practice," Zarif said, speaking in English at a joint press conference with Iraqi Foreign Minister Hoshyar Zebari.

"Why (do) they (countries supporting a strike) call themselves civilised nations and continue to insist on all options being on the table?" he said.

"All options have been removed from the table long, long, long time ago."

Zarif made the remarks during a one-day trip to Iraq, his first since being appointed foreign minister by President Hassan Rowhani in mid-August, and after meeting with Iraqi Prime Minister Nuri al-Maliki earlier on Sunday.

Zebari, meanwhile, warned that any strikes "would hinder political efforts" to help end the 30-month war in neighbouring Syria.

And in a rare cross-sectarian show of unity on the subject, Iraq's parliament speaker -- who is also the country's most senior Sunni Arab politician -- cautioned against military action.

"The military strike will not be beneficial towards Syria and will ignite a fire that will possibly extend to Iraq and nearby countries," Osama al-Nujaifi said in a separate news conference in Baghdad.

Nujaifi's comments were a rare criticism of the potential strikes by a Sunni Arab leader.

Iran, a staunch supporter of the Assad regime, actively opposes plans by the US and France to launch a military strike against Damascus over its suspected use of chemical weapons in deadly attacks on August 21.

It also backs claims in Damascus that rebels, not the Assad regime, carried out the chemical attacks on Damascus suburbs, which killed hundreds of people. Assad's regime has denied any responsibility.

Before leaving for Baghdad, Zarif was quoted by the official IRNA news agency as saying that Iran was "more worried" by the developments in Syria than other regional countries.

"The warmongering is happening in our neighbourhood, which is an important issue and has made my visit to Iraq necessary," he said.

Zarif seized on US President Barack Obama's failure to win support for military action against Syria from world leaders during the G20 summit in Saint Petersburg.

"This shows the US and pro-war groups are faced with definite isolation in their pursuit of using war and illegal means to push forward their own foreign policy agenda," he said in Tehran.

Iran provides Damascus with material and intelligence support but denies accusations of arming the Assad regime to fight the conflict which began as a popular uprising in 2011 but has now evolved into a fully-fledged civil war that has claimed more than 110,000 lives.

Iraq, meanwhile, has sought to publicly avoid taking sides in the civil war between Assad and rebels seeking his ouster, but the conflict has spilled over the border on several occasions, and has amplified divisions between Iraq's Sunni and Shiite communities.

Washington has repeatedly called on Iraq to stop flights allegedly carrying arms from Iran to the Syrian regime. Iraq insists Iran has reduced flights transporting arms to Syria but says Baghdad cannot stop them completely.

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