Russia has increased its deployment of warplanes to Syria but so far they appear to be positioned to defend their own base rather than to mount an offensive campaign, US Secretary of State John Kerry said Tuesday.

"Yes, they have increased aircraft and there are certain kinds of aircraft there which, depending on what Russia's decision is about its long-term intentions, could raise questions," Kerry said.

"But for the moment, it is the judgment of our military and most experts that the level and type represents basically force protection for their deployment to an airbase, given the fact that it is in an area of conflict."