There is a new crisis for the international effort to destroy Islamic State, created by the Kremlin. The United States has stopped flying manned air-support missions for rebels in a key part of northern Syria due to Russia's expansion of air-defence systems there, and the Obama administration is scrambling to figure out what to do about it.

Russia's military operations inside Syria have been expanding in recent weeks, and the latest Russian deployments, made without any advance notice to the US, have disrupted the US-led coalition's efforts to support Syrian rebel forces fighting against IS near the Turkey-Syria border, just west of the Euphrates River, several Obama administration and US defence officials said. This crucial part of the battlefield, known inside the military as Box 4, is where a number of groups have been fighting IS for control, until recently with overhead support from US fighter jets.

But this month, Moscow deployed an SA-17 advanced air defence system near the area and began "painting" US planes, targeting them with radar in what US officials said was a direct and dangerous provocation. The Pentagon halted all manned flights, although US drones are still flying in the area. Russia then began bombing the rebels the US had been supporting.

Inside the top levels of the administration, officials are debating what to do next. The issue is serious enough that Secretary of State John Kerry raised it with Russian President Vladimir Putin when they met on Tuesday, and US Joint Chiefs of Staff Chairman General John Dunford has discussed it with his Russian counterpart as well, a spokesman for US Air Force Central Command said.