Security forces have killed 18 people, including an 8-year-old boy, by shelling residential areas of the cities of Homs and Deraa and nearby villages, human rights activists reported Wednesday.
The authorities have also detained hundreds of people in what the New York Times has termed "one of the most brutal waves of repression since the Arab Spring began."
"Homs is shaking with the sound of explosions from tank shelling and heavy machine guns," activist Najati Tayara told Reuters.
Tayara told AFP that "shelling and automatic gunfire could be heard early Wednesday in the (Homs) neighbourhood of Bab Amr and in nearby villages.”
Homs has been a hub for anti-government protests against the rule of President Bashar al-Assad, and the city and neighboring villages have faced a crackdown by security forces since tanks moved in on the weekend.
Checkpoints have been placed at the entrances to the city, and water, electricity and communications lines have reportedly been cut.
Meanwhile, Ammar Qurabi, who heads the National Organization for Human Rights in Syria, told the Associated Press that 13 people were killed in tank shelling on a village outside Deraa, the southern city where the Syrian uprising began in mid-March.
Since Monday, the Syrian army has been sweeping the area looking for weapons, and there have been reports of theft and looting. "This operation terrified residents and security agents took part in looting," Tayara said.
The agricultural area has a population of about 100,000 people, many of them Bedouins, he said.
Thousands have been arrested and hundreds killed in the crackdown. The civilian toll estimated by various human rights groups ranges from 600 to 800 people. Foreign journalists are barred from entering the country and so it is impossible to independently verify reports.
The Syrian government says that it is pursuing "armed terrorist gangs".
The BBC reports that despite the crackdown on protesters, solidarity demonstrations are being held in many parts of the country. Mass demonstrations continued in the town of Jassem, north of Deraa, despite the army moving in tanks overnight.
UN Secretary General Ban Ki-moon on Wednesday again urged President Assad to desist from excessive use of force, stop the mass arrests and let in the UN.
"I urge again President Assad to heed calls for reform and freedom and to desists from excessive force and mass arrest of peaceful demonstrators," Ban told journalists.