At least 27 people were killed overnight as the Syrian army attacked three flashpoint cities, anti-government activists said Thursday, with tanks returning to the central city of Hama a day after they had reportedly been withdrawn.

The Federation of the Local Coordination Committees of the Syrian Revolution, an opposition group, said 19 civilians had been killed in the central city of Homs.

The other deaths occurred in the north-eastern city of Deir al-Zour and in Idlib near the border with Turkey, they said.

Activists posted footage on the internet showing a mosque minaret being toppled by gunfire in Deir al-Zour. They blamed the army and security forces.

Other footage showed what activists said were overnight anti-government protests in several Syrian areas, including Homs, the southern province of Daraa and in the suburban town of Hersta near Damascus.

Meanwhile, broadcaster Al-Jazeera quoted residents as saying that troops had not withdrawn from Hama, the scene of a 10-day military operation.

Turkish Prime Minister Recep Tayyip Erdogan said Wednesday the Turkish ambassador in Damascus had visited Hama and reported that tanks had started to leave the city.

The tanks returned to Hama after the end of the visit, during which the Turkish envoy was accompanied by journalists, according to activists.

The continuing crackdown came as international pressure on the government of President Bashar Assad mounted. Turkish Foreign Minister Ahmed Davutoglu delivered a "stern message" to Assad on Tuesday.

A U.S. diplomatic official told the New York Times the United States increasingly believed that Assad could not hold out indefinitely and that plans were being made for a post-Assad era.

"It's going to be messy," the unnamed official told the paper, raising concerns that a civil war could break out.

Around 1,730 civilians and 406 security personnel have been killed since the pro-democracy protests started in Syria in mid-March, the London-based Syrian Observatory for Human Rights said.

It is hard to verify these reports, as the Syrian authorities have barred most foreign media and international human rights groups from the country.