Syrian activists say government airstrikes killed at least 20 people as the army presses ahead with its campaign to crush the rebellion against President Bashar Assad.

The Britain-based Syrian Observatory for Human Rights says the strikes Sunday targeted the northern cities of Aleppo, Homs, Hama, Idlib, the western Mediterranean city of Latakia, the eastern province of Deir el-Zour and the suburbs of Damascus.

To the south in Daraa, a man was shot dead by an army sniper, the Observatory said. It added that there was little rebel advancement in the province where opposition forces seized large swathes of land over the past two weeks.

Meanwhile, the government urged rebels to surrender their arms, warning in cell phone text messages that the army is "coming to get you."