WASHINGTON — Russia is intensifying a pressure campaign on U.S. military forces in northeastern Syria following the American withdrawal from much of that area ahead of a Turkish cross-border offensive last fall, American military and diplomatic officials say.

Russian military personnel have increasingly had run-ins with U.S. troops on highways in the region, breaking agreements between the two countries to steer clear of each other. Russian helicopters are flying closer to American troops. And on Wednesday, a U.S.-led convoy returned fire after it came under attack near a checkpoint manned by forces loyal to President Bashar al-Assad of Syria, who are backed by Russia.

American officials say these actions by Russian personnel and their Syrian allies are devised to present a constant set of challenges, probes and encroachments to slowly create new facts on the ground and make the U.S. military presence there more tenuous. About 500 American troops remain deployed in Syria with a mission to protect oil fields and help fight remnants of the Islamic State.

“These are not daily occurrences but they have been increasing in number, and thus is troubling,” James F. Jeffrey, the top American diplomat overseeing Syria issues, told reporters last week.