Britain joining strikes against jihadists has not been requested and is not currently under discussion, Downing Street has insisted, despite reports that Barack Obama is hoping to win agreement to bring allies into the air campaign by next week's Nato summit.
The United States has launched scores of bombing attacks on Islamic State (IS) militants in northern Iraq in a bid to assist Kurdish and Iraqi forces in their fightback.
Reports in The Times said the Pentagon had been exploring whether western allies such as Britain and Australia, and allied Gulf states, would assist in a broader campaign in Syria against the group, which was formerly known as Isil.
But a No 10 spokeswoman said: " There's been no request for us to deliver air strikes and this is not something under discussion at the moment.
"Our focus remains on supporting the Iraq government and Kurdish forces so that they can counter the threat posed by Isil, for example with the visit of our security envoy to Iraq this week and the provision of supplies to Kurdish forces."
The report suggested US president Mr Obama asked the Pentagon to carry out a "scoping exercise" with allies to discover their approach to joining a campaign.
Nato members are due to gather at Celtic Manor, south Wales, on September 4 and 5, for a summit.
The Commons rejected British bombing in Syria in a historic vote almost exactly a year ago when Prime Minister David Cameron sought approval for military strikes in response to chemical attacks.
And The Times reported scepticism about whether domestic politics would allow Britain to become involved.
An unnamed Conservative minister told the paper: "David Cameron is simply not going to want to get involved this c lose to the election, even though it's the right thing to do. The risks are too big."
A Whitehall source also questioned the idea, saying: "The idea that we could somehow do military action in Syria without a parliamentary vote when there has already been a parliamentary vote disallowing it, it's just not going to happen."
Any action in Syria would go ahead without the permission of the Assad regime, raising the risks involved.