The British-Based Syrian Observatory for Human Rights and the Local Coordination Committees said the troops entered Saraqeb from the north and were pushing forward. The Observatory said at least one civilian was killed in the attack.
The LCC said the troops were accompanied by pro-government gunmen known as shabiha and plainclothes security agents who arrived in buses and started conducting raids and detaining people. Calls to the city could not get through. The government is known to cut networks in areas where operations are underway. Saraqeb, in the northern province of Idlib that borders Turkey, has been held by army defectors for months.
The attack on Saraqeb came 11 days after troops retook Idlib city, the provincial capital, which had been under rebel control for months. President Bashar Assad's forces have been on a series of offensives over the last month, and in the last two weeks have targeted rebel-held areas in Idlib province and in the central city of Homs.
International condemnation and high-level diplomacy have failed to stop the year-old Syria crisis that the U.N. says has killed more than 8,000 people, many of them civilian protesters.
Earlier this week, a U.N. Security Council statement called for a cease-fire to allow for dialogue between all sides on a political solution. Assad's government played down the statement, saying Damascus is under no threats or ultimatums.
The Syrian uprising began last March with protests calling for political reforms. Unrest spread as Assad's forces violently tried to quash dissent, and many in the opposition took up arms to defend their towns and attack government troops.
Earlier Saturday, the Observatory and the LCC said troops fired mortar rounds at the rebel-held neighborhood of Khaldiyeh in the central city of Homs in apparent preparation to storm the area.
Rami Abdul-Rahman, who is head of the Observatory, said the heavily-populated Khaldiyeh has been shelled since early Saturday. The LCC posted a video on its Facebook page showing smoke billowing from a residential area it said was in Khaldiyeh.
The neighborhoods, one of Homs' largest, has been under rebel control for months.
Homs has seen some of the heaviest fighting in Syria's year-long uprising. Government forces crushed a rebel stronghold in Baba Amr neighborhood on March 1.