Syrian troops in tanks and armoured vehicles poured into the southern town of Deraa and opened fire today, residents said, in the latest bloodshed in a crackdown on protests that has escalated sharply in recent days.
At least 18 people were killed, prominent activist Ammar Qurabi said.
He said some were killed by gunfire and others when tanks shelled buildings in the city. Many others were wounded or missing, he said.
As the army tried to regain control of Deraa, where a month-long uprising against President Bashar al-Assad erupted last month, campaigners said forces also stormed the Damascus suburb of Douma, which has seen large anti-Assad protests.
The top United Nations human rights official called on Syria today to rein in its security forces and investigate the reported killing of 100 protesters over the weekend.
UN High Commissioner for Human Rights Navi Pillay also urged President Bashar al-Assad and his government to implement promised reforms and release detained activists and political prisoners.
"The first step now is to immediately halt the use of violence, then to conduct a full and independent investigation into the killings, including the alleged killing of military and security officers, and to bring the perpetrators to justice," Ms Pillay, a former UN war crimes judge, said in a statement.
Meanwhile, the White House said today that it is considering "targeted sanctions" against the Syrian government.
"The brutal violence used by the government of Syria against its people is completely deplorable," White House spokesman Tommy Vietor said. "The United States is pursuing a range of possible policy options, including targeted sanctions, to respond to the crackdown and make clear that this behavior is unacceptable."
A US official said earlier that the measures under consideration included a freeze on assets and a ban on U.S. business dealings.
Rights groups say security forces have killed more than 350 civilians since unrest began. A third of the victims were shot in the past three days as the scale and breadth of a popular revolt against Assad grew.
Assad lifted Syria's 48-year state of emergency on Thursday but activists say the violence the following day, when 100 people were killed during protests across the country, showed he was not serious about addressing calls for political freedom.
Today's raids on Deraa and Douma appeared to show that Assad, who assumed power when his father died in 2000 after ruling Syria with an iron fist for 30 years, was determined to crush the opposition with force.
Residents in Deraa said hundreds of troops had arrived.
One witness told Reuters he could see bodies lying in a main street near the Omari mosque after eight tanks and two armoured vehicles deployed in the old quarter of the city.
"People are taking cover in homes. I could see two bodies near the mosque and no one was able to go out and drag them away," the witness said.
Snipers were posted on government buildings, and security forces in army fatigues had been shooting at random at houses since the tanks moved in just after dawn prayers.
"They were firing. Witnesses have told me that there have been five deaths so far and houses have become hospitals," another Deraa resident named Mohsen told Al Jazeera, which showed what appeared to be a cloud of black smoke over the town.
Tanks at the main entry points of the city were also shelling targets in Deraa, Mohsen said. "People can't move from one street to another because of the shelling."
Foreign journalists have mostly been expelled from the country, making it impossible to verify the situation on the ground. Grisly footage posted on the Internet by demonstrators in recent days appears to show troops firing on unarmed crowds.
Officials have blamed armed groups for the violence.
Syria also closed all its land border crossings with neighbouring Jordan today.
A senior diplomat in the Jordanian capital confirmed that the two main Syrian crossings at Deraa and Nassib on the Syrian side were closed to traffic. An official told Reuters the "timing is related to what appears to be a major security operation that is taking place right now".
Meanwhile, a leading Syrian human rights campaigner said today that the Syrian authorities have launched a war against the country's peaceful pro-democracy movement by attacking three cities.
"This is a savage war designed to annihilate Syria's democrats," Suhair al-Atassi said in a statement sent to Reuters.
"President Bashar al-Assad's intentions have been clear since he came out publicly saying he is 'prepared for war' in his speech on March 30. I am at my home in Dummar suburb of Damascus. Come and arrest me," she said.