"Clearly, the presence of aircraft with air-to-air combat capacity as well as ... surface-to-air missiles raises serious questions," Secretary of State John Kerry said in London in response to a question from CNN.

Four Russian Sukhoi "Flanker" jets arrived Friday in Syria, according to a U.S. official. It's unclear how the aircraft will be used, but Russia has said its military buildup in Syria is to support Bashar al-Assad's regime and combat ISIS.

Before that development, the United States had observed eight Russian military helicopters inside Syria, and seen Russia move tanks, mobile artillery and armored vehicles into the country.

What is Russia up to in Syria?

The addition of Russian combat jets raises fears of an accidental encounter as U.S. and coalition aircraft fly over Syria to combat ISIS targets.

After a meeting with his British counterpart, Kerry said the movement of fighter jets into Syria is "precisely why" Defense Secretary Ash Carter and Russian Defense Minister Sergei Shoygu spoke Friday.

"We are engaged in further conversation about answering those questions and about de-conflicting the Russian activities from ours," Kerry said.

The Carter-Shoygu conversation was the first since the U.S. defense chief took his position. Military discussions between the United States and Russia were suspended in the wake of Russia's annexation of Crimea and involvement inside Ukraine.

CNN's Barbara Starr and Jamie Crawford contributed to this report.