UNITED NATIONS: Washington's arch-foe Iran has a role to play in tackling Islamic State (IS) militants who have overrun large swathes of territory in Iraq and Syria, US Secretary of State John Kerry said.

Iran's supreme leader Ayatollah Ali Khamenei said this week that his government had rejected a request from the United States for cooperation on the battlefield.

US officials have not confirmed or denied making an offer in private, but they do not regard Iran as part of the coalition they are building to fight the IS militant scourge.

Kerry said Friday that in combating the jihadist threat “there is a role for nearly every country to play, including Iran.”

Just last week, the top US diplomat had said it would be inappropriate to invite Iran to meetings seeking to building an international coalition because of Tehran's “engagement in Syria and elsewhere.”

Iran is actively supporting Iraq's government against IS jihadists, and helping Syria in its battle against both the IS group and unaffiliated rebel groups.

Read more: World will be at Iran, Syria’s mercy if US fails to crush IS, warns Kerry

Washington has struggled with its anti-IS engagement in Syria, already in the grips of a brutal civil war pitting the rebels against the forces of President Bashar al-Assad.

Speaking before a group of 35 countries brought together for a meeting at the UN Security Council by top US diplomat John Kerry, Iran's deputy foreign minister, Abbas Araghchi, stressed the need to cooperate with governments in the region.

“In order to fight IS in Iraq and Syria, we must cooperate with the governments of those countries,” he said.

“Any military strategy, especially as concerns Syria, that would weaken that country's central government is doomed to fail.”

Tehran is a close ally of Damascus, and Araghchi insisted Iran was “the only country in the region that has supported the Iraqi government in its fight against IS.”

“Iran is ready to help the government of Iraq and other countries in the region facing the IS threat,” Araghchi added in comments reported by the IRNA state news agency.

He later told Iranian media in New York that both the Syrian and Iraqi governments had to be helped with equal measure.

“In the fight against terrorism, the Iraqi and Syrian governments must be supported and any military action against this terror group must take place according to international law and the United Nations Charter,” Aragchi said.