“We have regularly and repeatedly expressed our concern about Russian military support for the Assad regime,” said John Kirby, the State Department spokesman. “But we’re also watching their actions very carefully. If these reports are borne out, it would represent a very serious shift in the trajectory of the Syria conflict and call into question any Russian commitment to a peaceful settlement.”

Mr. Kerry flew to Sochi, Russia, in May to meet with Mr. Putin to explore whether the two sides could cooperate on Syria. In August, Mr. Kerry followed up with an unusual three-way meeting in Qatar on the Syria crisis with Sergey V. Lavrov, the Russian foreign minister, and their Saudi counterpart, Adel al-Jubeir.

Stepped-up Russian military support for the Syrian government could pose a problem for the United States in several ways. If Mr. Putin’s intention is to support not just the Syrian government but also Mr. Assad, that could undercut Mr. Kerry’s contention that the Syrian president needs to leave power as part of any political solution to the conflict.

And if Russian pilots carried out airstrikes, administration officials say, the choice of targets might further aggravate the growing chaos. Russian strikes on Islamic State militants could interfere with, or at least complicate, the air operations that the United States-led coalition is already conducting in Syria against the group. But if Russia targets rebel groups that are opposed to Mr. Assad, they might be striking some of the moderate Syrian fighters who have been trained by the C.I.A. and the Pentagon.

Another possibility is that Russia is taking these steps to secure its own interests in the event that the Assad government collapses, reaches a power-sharing agreement with the opposition or is replaced. By expanding its military influence in Syria, Russia might be in a stronger position to shape the political outcome as the Assad government’s military position weakens by encouraging it to share power with opposition members Moscow supports.

It is not clear if the Obama administration has pressed the Russian government on its intentions, although some officials said such a discussion was likely in the coming days.