Asked if the development could result in sending troops into Syria, Cameron said he didn't want to see that.
"But I think we can step up the pressure on the regime, work with our partners, work with the opposition in order to bring about the right outcome," he said, according to the Press Association. "The question is how do we step up the pressure. And, in my view, what we need to do -- and we're doing some of this already -- is shape that opposition, work with them, train them, mentor them, help them, so that we put the pressure on the regime and so what we can bring this to an end."
The Syrian government has been battling a rebellion for more than two years, bringing international condemnation of the regime and pleas for greater international assistance.
The United Nations estimated in February that more than 70,000 people had died since the conflict began.
The opposition Local Coordination Committees in Syria reported that 139 people, including 16 women and 14 children, had been killed across the country on Friday. Twenty-nine of those deaths were in and around Damascus, while 27 were in Homs province.
Members of the rebel Free Syrian Army clashed with government forces in at least 115 places around Syria on Friday, during which the opposition group reported 235 bombing attacks -- including from warplanes and surface-to-surface missiles.