The US has declared that it is at war with the Islamic State group, a day after Barack Obama said strikes against the fighters would extend into Syria.

The comments by Pentagon and White House officials on Friday came as the US president was pressed to clear up doubts about how he saw the conflict with the armed group, also known as ISIL.

"The United States is at war with ISIL in the same way that we are at war with al-Qaeda," said Josh Earnest, a spokesman for the White House.

John Kirby, a spokesman for the defence department, echoed Earnest's remarks: "We know we are at war with ISIL," he said.

The comments came a day after US secretary of state, John Kerry, appeared to be reluctant to use the term.

"I think 'war' is the wrong terminology and analogy but the fact is that we are engaged in a very significant global effort to curb terrorist activity," he said.

Kerry on Friday travelled to Turkey to discuss a broad coalition to counter the Islamic State group in Syria. However, when asked, he ruled out Iranian involvement as inappropriate due to its support for the regime of Bashar al-Assad.

He said there were "any number of reasons" why Iran should not join a Paris conference on Monday to discuss coalition strategy.

"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" in Paris.

Iran has never offered to join the coalition against ISIL in Syria, describing it as "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.

Iran is believed to have sent troops to support the 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 won backing on Thursday for a "coordinated military campaign" against ISIL from 10 Arab countries - Egypt, Iraq, Jordan, Lebanon and six Gulf states including Saudi Arabia and Qatar.

Ankara has refused to allow the US to use its main airbase in Turkey to launch attacks on Syrian soil.