A military breakthrough may first require diplomatic agreement on the formation of a new transitional government capable of uniting warring forces to drive IS from northern Syria, government sources added, the report says.
"We are going to need to be patient and persistent. This is going to take time. It is complex, it is difficult what we are asking our pilots to do, and our thoughts should be with them and their families," David Cameron said.
Michael Fallon, the defense secretary, said the air campaign, due to be supported by reconstituted Syrian troops, could last years. "The American estimate of the campaign in Iraq, which began last year, was that it would last at least three years and we're not halfway through that yet."
Philip Hammond, the foreign secretary, is due to attend a series of talks this month that will determine if the countries shaken by recent IS attacks can bury their differences over Syria and bring an end to the four-year civil war.
The Foreign Office also hopes to press ahead with an ambitious timetable to win agreement this month for the launch of UN-sponsored talks to end the civil war.