BEIRUT (Reuters) - Syrian rebels killed 21 elite Republican Guards on Thursday in an ambush on an army minibus in a suburb northwest of Damascus, opposition activists said.

The Syrian Observatory for Human Rights quoted a medical source and witnesses as saying the troops had been killed in Qudsayya, which has been shelled by forces loyal to President Bashar al-Assad after rebels moved into the area.

The Republican Guards are specifically entrusted with defending the capital, and the army stepped up operations in Qudsayya on Wednesday morning, Damascus residents said. They reported hearing loud explosions and said that a large military convoy was heading to the area.

Assad used force of arms to try to crush a peaceful pro-democracy movement when it erupted in March 2011. He now faces a full-scale armed revolt that has brought rebels into the suburbs of Damascus.

The army has responded with artillery and air strikes, even though many residents have been unable to flee.

An opposition activist who lives near Qudsayya said that Assad's forces had blocked the roads on Thursday and civilians were stuck in the area.

"Shelling started again this morning. I had a friend who managed to escape last night through back roads. He said regime forces were storming the area and had killed five people," she said over Skype, requesting anonymity.

"Those who are still there are trying to get out, but they are afraid of snipers."

(Reporting by Oliver Holmes; Editing by Kevin Liffey)

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