Hardline Islamists are among the so-called moderate opposition forces on the ground in Syria, David Cameron has conceded.
Critics have questioned the number of non-extremist fighters after the Prime Minister claimed there were 70,000 in the country who could form the basis of the ground force against Bashar Assad's regime.
Mr Cameron insisted he stood by his claim - based on the advice of the Joint Intelligence Committee - but told a committee of senior MPs there were not enough overall and admitted that some were "relatively hardline Islamists".
He told the Liaison committee: "Some of the opposition forces are Islamist. Some of them are relatively hardline Islamist."
"They are not all the sort of people you would bump into at a Liberal Democrat party conference," he added.
Mr Cameron said "deep tensions" between Iran and Saudi Arabia mean peace talks to bring the Syrian civil war to an end will be "incredibly difficult".
Britain will encourage moderate opposition groups to attend the next round of talks on January 25, when US Secretary of State John Kerry hopes rebels will sit round a table with representatives of Bashar Assad's regime in the hope of a ceasefire.
But he acknowledged that opposition from Turkey means there may be no place at the table for Syrian Kurds, who have provided many of the most effective military forces fighting both Assad and the Islamic State terror group (IS) - also known as Daesh, Isil and Isis.
He insisted that it was possible to find a "third way" solution that did not involve leaving the country in the hands of Assad or the hands of IS.
"It is impossible in my view to really envisage a situation in which Assad stays in power and Syria isn't a threat to our national interest," he added.
Mr Cameron told the committee he was unlikely to allow all the information surrounding the events that led to the drone strike that killed two British nationals fighting with IS to be released to MPs investigating the incident.
Parliament's intelligence watchdog is looking into the decision to authorise the attack on 21-year-old Reyaad Khan from Cardiff, which also killed a second British fighter, Ruhul Amin.
In terse exchanges, committee chairman Andrew Tyrie said the Prime Minister was not giving the Intelligence and Security Committee (ISC), the information it needs, claiming the premier had "excluded" details about the military circumstances around the strike.
Mr Cameron said his "instinctive answer would be no" to demands for all information to be made available to the ISC and claimed such a move could risk the lives of intelligence sources.
The premier also said he was "not inclined" to publish policy on the use of drones strikes to kill terrorists because he "could see some disadvantages" in such a move, including giving away intelligence information to terror groups.
Mr Tyrie also criticised the Prime Minister for the response in Libya in the wake of the fall of the Gaddafi regime, claiming the "humanitarian balance sheet of this intervention doesn't look good".
Mr Cameron insisted Britain had been involved in nation building but conceded "on this occasion, clearly it didn't work."