Russia and Iran have stepped up coordination inside Syria as they move to safeguard President Bashar al-Assad’s control over his coastal stronghold, according to officials in the U.S. and Middle East, creating a new complication for Washington’s diplomatic goals.

Senior Russian and Iranian diplomats, generals and strategists have held a string of high-level talks in Moscow in recent months to discuss Mr. Assad’s defense and the Kremlin’s military buildup in Syria, according to these officials.

The...