Other American officials suggested, however, that the deployment might simply reflect the Russian military’s standard defensive precautions as it established an air hub in a foreign country.

The prefabricated building Russia has erected at the base now has the capacity to house 2,000 military advisers and personnel. Ferrying weapons and equipment to the base has involved well over 20 flights by Russian Condor transport planes — almost all of which have flown to Syria by passing over Iran and Iraq.

Syria, and the migrant crisis it has spawned, has been a major focus of Mr. Kerry’s trip to Europe. After a meeting Saturday morning with Philip Hammond, the British foreign secretary, Mr. Kerry said that it was vital to pursue a diplomatic solution to the crisis but that Moscow was not putting enough pressure on President Bashar al-Assad of Syria to make him negotiate seriously.

“We need to get to the negotiation,” Mr. Kerry said at a joint news conference with Mr. Hammond. “That’s what we’re looking for, and we hope Russia and Iran, other countries with influence, will help to bring that about, because that’s what’s preventing this crisis from ending.”

“Right now, Assad has refused to have a serious discussion,” Mr. Kerry added, “and Russia has refused to help bring him to the table in order to do that.”

Mr. Hammond and Mr. Kerry each emphasized that Mr. Assad could not remain in power if there was to be a durable solution to the conflict, but they said that the timing of his departure during a political transition in Syria would be a matter of negotiation.

“It doesn’t have to be on Day 1 or Month 1,” Mr. Kerry said. “There is a process by which all the parties have to come together and reach an understanding of how this can best be achieved.”