US officials have requested that Bulgaria and Greece close their airspace to Russian supply flights.
The new vow adds fuel to fiery debate in Britain. Cameron said Monday that a strike in Syria against suspected Islamic State militants in August had killed two British citizens.
Turkey has vowed to step up its campaign against the outlawed PKK. The collapse of its cease-fire with the PKK has complicated the US-backed campaign against the self-proclaimed Islamic State.
Russia is a key backer of the Assad regime. Western powers have insisted that Assad must step aside under any peace agreement.
The US-led military alliance against Islamic State has carried out multiple airstrikes over the past year. The Washington Post reported Tuesday that a separate drone campaign is under way against leaders of the militant group.
Fighting between government forces and rebels has killed more than two dozen people in and near Damascus.
Hundreds of thousands of immigrants, fleeing violence and poverty in Africa and the Middle East, are trying to find a better life in Europe – putting major strains on the EU and its member states.
Even as the US and Turkey agree on trying to drive the Islamic State away from the Syrian side of the border with Turkey, there are as many agendas as there are players trying to shape the conflict.
Turkey has long sought a buffer zone in northern Syria. Critics say it is exploiting the US-led battle against Islamic State militants to strike at Kurdish forces in Iraq and Syria that it sees as a threat to Turkish rule.
The airstrikes were prompted by intelligence indicating a likely Islamic State attack. The US said a deal has been reached with Turkey to use a key airbase.
Little is known about the Khorasan group, though it is said to be a small, elite group of Al Qaeda members involved in planning high-level attacks on the US.