BEIRUT, Lebanon — Security forces massed on Sunday near a key town in eastern Syria where dozens of soldiers have defected to join the four-month-old uprising against the government of President Bashar al-Assad, according to activists and residents there.

At least 1,000 troops, some backed by tanks and others flown in on helicopters, descended on the town, Albokamal, which is near the border with Iraq and that nation’s Sunni Muslim heartland.

The Syrian forces surrounded the town and appeared to be preparing for a major military operation to quell dissent there, residents said.

Al-Watan, a pro-government newspaper, reported on Sunday that the situation in Albokamal was “explosive” and that the army was “preparing to intervene.”