NEW YORK CITY (AFP) - Iranian President Hassan Rohani warned on Thursday that Syria could become an extremist haven like Taleban-era Afghanistan as he called for cooperation to end the country's civil war.

Iran considers Syrian President Bashar al-Assad its closest regional ally and has not accepted US intelligence that the regime killed some 1,400 people in a chemical weapons attack last month.

"My government strongly condemns the use of chemical weapons in Syria," Mr Rohani told a New York think tank forum, without assigning blame.

"I am also concerned about the breeding ground created in parts of Syrian territory for extremist ideology and a rally point for terrorists, which is reminiscent of another region adjacent to our eastern borders in the 1990s," he said.

"This is an issue of concern not only to us but also to many other countries, which requires cooperation and joint efforts aimed at finding a durable, inter-Syrian political solution."

Iran, led by a Shiite theocracy, opposed the 1996-2001 rule in Afghanistan of the Taleban, who welcomed Al-Qaeda militants and enforced an austere brand of Sunni Islam.

The secular-minded Assad belongs to the heterodox Alawite community and is battling rebels who include Sunni hardliners.

In an earlier false start of better ties, Iran and the United States briefly cooperated in 2001 when a US-led campaign ousted the Taleban.

Mr Rohani welcomed a US-Russian agreement for Mr Assad to give up chemical weapons, which halted a push for a military strike on Syria by US President Barack Obama.

"We are pleased that diplomacy... and sober judgment prevailed over saber-rattling," Mr Rohani said.

Iran itself was a victim of chemical weapons used by Iraq, a history acknowledged by Mr Obama in his address to the United Nations earlier this week.

Western powers largely supported Iraqi president Saddam Hussein in his 1980-88 war against Iran.

Mr Rohani, elected in June on a platform of moderation, is on a visit to the United Nations aimed largely at easing tensions with Western powers over Iran's contested nuclear programme.

US Secretary of State John Kerry held a brief but historic meeting on Thursday with Iranian Foreign Minister Mohammad Javad Zarif, who said that he agreed with major powers to seek a deal within a year to resolve concerns over Teheran's nuclear programme.

But despite the upbeat tone, the United States was cool to letting Iran participate in a peace conference in Geneva on ending Syria's civil war, the next step after a UN Security Council resolution enforcing the US-Russian chemical weapons agreement.

A senior US official, speaking on condition of anonymity, said that Iran first had to commit to the declaration of the first Geneva conference last year that called for a political transition in which Mr Assad would leave.

"Anyone who wants to participate has to, at the very least, sign up to the Geneva communique and Iran has not explicitly done so," the official said.