The US has not asked Britain to take part in airstrikes in Syria and it is “not something under discussion”, according to a Downing Street spokeswoman.
Barack Obama is gearing up for strikes on Islamic State militants in the country, according to reports, and is looking to recruit allies to help.
But Number 10 denied Britain had been asked to take part.
“There’s been no request for us to deliver airstrikes and this is not something under discussion at the moment,” said a spokeswoman.
“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 Times reports that the US is looking at attacking the terror group with assistance from countries such as the UK, Australia and allied Gulf states.
It reports that a “scoping exercise” is under way to gauge support.
The issue is set to be high on the agenda when Nato members meet next week at Celtic Manor in south Wales for a two-day summit.
MPs last year voted against airstrikes against Syria’s President Assad, after Western powers accused him of using chemical weapons in the country’s civil war.
Any decision to join airstrikes against IS would be likely to face the same hurdle.
The beheading of US journalist James Foley by the Islamic State has piled pressure on President Obama to take tougher action against the group.
American strikes on IS fighters continue in Iraq, with British planes helping with surveillance.
But the group remains strong in the east of Syria – one of the countries where it has set up a self-proclaimed caliphate.
Its advance has seen it seize large swathes of land in the region, reportedly massacring enemies and non-Muslims.