Syrian rights organisation Sawasiah said it had received reports that at least 20 people had been killed in Deraa since tanks moved in on Monday, but communications with the southern town where the protests against President Bashar al-Assad began on 18 March had been cut, making it hard to confirm the information.
At least 500 were arrested elsewhere in Syria, it said.
Diplomats said the figures for civilians killed could be as high as 50 in Deraa and 12 in Mouadhamiya, which lies on the road to occupied Golan Heights southwest of Damascus.
"The regime has chosen to use excessive violence. It worked in 1982, but there is no guarantee it will work again in the age of the Internet and phone cameras," said a senior diplomat, referring to the 1982 crushing of a revolt in the city of Hama, which killed up to 30,000 people.
Footage posted on the Internet by demonstrators in recent days appears to show troops firing on unarmed crowds. In the Damascus suburb of Barzeh residents described security forces firing at unarmed protesters from a heavy machinegun mounted on a truck.
"By resorting to the use of artillery against its own people today, the Syrian government has shown its determination to crush the peaceful protests at virtually any cost, whatever the price in Syrians' lives," said Malcolm Smart, Amnesty International's Middle East and North Africa director.
Government forces also stormed the Damascus suburbs of Douma and Mouadhamiya on Monday, shooting and making arrests, a day after they swept into the coastal town of Jabla, where at least 13 civilians were killed, rights campaigners said.
Ashton condemns violence, urges reform
EU High Representative for Foreign Affairs and Security Policy Catherine Ashton condemned the violence and called on the Syrian government to carry out "profound political reforms".
"I condemn the use of brutal force against demonstrators across Syria, which is resulting in high numbers of victims. The Syrian authorities must immediately stop their violent response and fully respect citizens' right to peaceful demonstrations," Ashton said in a statement published on Friday.
The White House, deploring "brutal violence used by the government of Syria against its people," said President Barack Obama's administration was considering targeted sanctions to make clear that "this behaviour is unacceptable".
Syria has been under US sanctions since 2004 for its support of militant groups. Several Syrian officials, among them Assad's cousin Rami Makhlouf, a tycoon, are under specific US sanctions for "public corruption".
According to analysts, Obama's response to the Syrian crisis has been limited compared to Washington's role in a NATO-led air campaign against Libyan leader Muammar Gaddafi's forces and its call for his ousting.
Washington is mindful of its limited ability to influence Damascus, which is already under a set of US economic sanctions and is closely allied with US foe Iran.
The Obama administration is also worried about stoking instability on US ally Israel's borders and wants to avoid another military entanglement in the Muslim world, where it is involved in wars in Iraq and Afghanistan.
Strong measures against Syria by the UN Security Council, similar to those against Libya, are seen as unrealistic, due to the long-standing strong ties between Damascus and Moscow, Euronews reported.
(EurActiv with Reuters.)