US opposes Iran role in coalition against Islamic State
US Secretary of State John Kerry has said it would be inappropriate for Iran to join a coalition that is seeking to fight Islamic State (IS) militants.
Speaking on a visit to Turkey, Mr Kerry said he was confident the US could build a broad international coalition, of European and Arab countries.
Both Iran and the US have offered military aid to hold back an IS advance across northern and western Iraq.
But the US has clashed with Iran on its nuclear programme and policy in Syria.
Earlier this week, US President Barack Obama unveiled plans for an expansion of the campaign against IS in the region.
Ten Arab nations have agreed to help the US in its fight against the group, which the CIA says may have up to 31,000 fighters on the ground.
France has also offered its support for military action against IS, as part of a coalition being formed by Washington.
Paris is expected to host international talks on the campaign on Monday.
At a press conference in Ankara on Friday, Mr Kerry said he had not formally been asked to discuss "the presence of Iran" at the Paris conference.
"But I think under the circumstances at this moment in time... it would not be appropriate given the many other issues... with respect to their engagement in Syria and elsewhere," he said.
Iran has backed the government of President Bashar al-Assad in Syria, while the US and several European and Gulf countries have supported the rebel factions fighting to overthrow him.
The US and Western countries are also in talks with Iran over its nuclear programme, which they fear could be used to develop a bomb, allegations Iran has strenuously denied.
On Friday, Mr Kerry held talks with Turkish President Recep Tayyip Erdogan and Prime Minister Ahmet Davutoglu in an effort to secure more co-operation from the Turkish government in the fight against IS.
Turkey has refused to allow the use of its air bases to launch attacks on the jihadist group.
The BBC's Jim Muir in Irbil says one reason is that Turkey fears for the lives of nearly 50 Turkish hostages held by the militants, including staff from the consulate in Mosul.
After the meeting, Mr Erdogan's office said in a statement quoted by AFP news agency: "The two countries will continue to fight against the terrorist organisations in the regions as in the past."
It added that Turkey would continue to share intelligence with the US and provide logistical support to the Syrian opposition, as well as humanitarian aid to Syrians affected by the conflict.
On Thursday Mr Kerry met representatives from 10 Arab nations in Jeddah, Saudi Arabia, who said in a communique that they "agreed to do their share in the comprehensive fight" against IS.
A Pentagon spokesman said Washington was preparing a more "aggressive" air campaign, with some of the aircraft taking off from the air base at Irbil, in Iraqi Kurdistan.
President Obama on Wednesday said for the first time that he had also authorised air strikes against IS in Syria.
In recent months IS has expanded from its stronghold in eastern Syria and seized control of more towns, cities, army bases and weaponry in Iraq.
The US has already carried out more than 150 air strikes in northern Iraq. It has also sent hundreds of military advisers to assist Iraqi government and Kurdish forces, but has ruled out sending ground troops.