John Kerry has said it was "not appropriate" for Iran to join talks on confronting the Islamic State group in Syria due to its support for the regime of Bashar al-Assad.
The US secretary of state said on Friday in Turkey that support was "any number of reasons" why Iran should not join a Paris conference on Monday to discuss a coalition strategy against the Islamic State group, also called ISIL.
Live Box 2012108933861442
"Under the circumstances, at this moment in time, it would not be right for any number of reasons. It would not be appropriate given the many other issues that are on the table in Syria and elsewhere," he said in Ankara.
Kerry said he had not formally been asked to discuss "the presence of Iran" at the Paris conference.
Iran has never offered to join the coalition against ISIL in Syria, describing it as a coalition "shrouded in serious ambiguities". It has sent soldiers to Iraq at the request of the Iraqi government to combat ISIL there.
The US says it is not cooperating militarily with Iran in operations in Iraq.
Shia-led Iran supports the government of Iraq, which is also Shia-led, and is believed to have sent troops to support a Syrian regime led by Alawites, a Shia sect.
The US backs the Iraqi government but is opposed to the Syrian regime, and Iran's role in the civil war.
Kerry travelled to Turkey to try to secure backing for US-led action against ISIL, but Ankara has refused to allow the US to use its main airbase in Turkey to launch attacks on Syrian soil.
Kerry won backing on Thursday for a "coordinated military campaign" against Islamic State from 10 Arab countries - Egypt, Iraq, Jordan, Lebanon and six Gulf states including Saudi Arabia and Qatar.
But it remains far from clear what role individual nations will play.
"It is entirely premature and frankly inappropriate at this point in time to start laying out one country by one country what individual nations are going to do," said Kerry, who travels to Cairo on Saturday, adding that building a coalition would take time.
"I'm comfortable that this will be a broad-based coalition with Arab nations, European nations, the United States, others," he said. "At the appropriate time, every role will be laid out in detail."
Source: Al Jazeera and agencies