|Footage posted on YouTube appears to show Syrian troops in Deraa but Al Jazeera cannot verify its veracity
More deaths have been reported in Syria where thousands of troops backed by tanks and heavy armour have swept into the volatile town of Deraa in the south of the country and the large Damascus suburb of Douma.
Security forces also continued a crackdown in the coastal town of Jableh for a second day.
Al Jazeera's Rula Amin, reporting from Damascus, said the government's offensive on Monday was an "unprecedented" offensive against the wave of dissent that has swept the country since the uprising began on March 15.
Witnesses in Deraa told news agencies on Monday that at least five people had been killed when gunmen opened fire on a car.
The vehicle was riddled with bullets, a witness told AFP. Intense gunfire could be heard reverberating across the town, he added.
"The minarets of the mosques are appealing for help. The security forces are entering houses. There is a curfew and they fire on those who leave their homes. They even shot at water tanks on roofs to deprive people of water," he said.
Al Jazeera is unable to confirm the reported deaths. Thousands of soldiers swept into the town in the early hours of Monday, with tanks taking up positions in the town centre and snipers deploying on rooftops, witnesses said.
"Bodies are lying in the streets and we can't recover them," one activist said, explaining that they have little idea of the total number of casualties. Footage aired by an opposition news organisation on Monday, transmitted via satellite, appeared to show Syrian military firing at unseen targets with sniper rifles. Al Jazeera is unable to verify the veracity of the footage.
Security forces loyal to Bashar al-Assad, the country’s president, also stormed Douma early on Monday, shooting at unarmed civilians and arresting residents, rights campaigners said.
"There are injured people. Scores have been arrested. The security are repeating the same pattern in all the centres of the democratic uprising. They want to put down the revolution using the utmost brutality," an unidentified rights campaigner in Damascus told the Reuters news agency.
Al Jazeera's report showing video of shooting in Deraa province on Sunday
Douma has been the site of many large protests since the uprising against Assad began in Syria earlier in the year.
And in Jableh, security forces continued a crackdown that began the previous day.
"Jableh is surrounded by security forces," the witness said, speaking by telephone. "The dead are in the mosques and the houses. We can't get them out."
At least 13 people had been killed in Jableh since Sunday, the Syrian Observatory for Human Rights said on Monday morning.
Syria closed all border crossings on its southern frontier with Jordan as the military launched its operation in nearby Deraa, Al Jazeera learned from a security offical.
Amin reported checkpoints and heavy security in central Damascus.
Syria has banned nearly all foreign media and restricted access to trouble spots since the uprising began, making it nearly impossible to get independent assessments.
Al Jazeera's correspondent said that the events on Monday marked a change in methods by security forces.
Up until now, she said, security forces have cracked down in reaction to protests. But the flood of troops into Douma and Deraa had come in the absence of any demonstrations.
"Today, we're seeing a different tactic with security forces sweeping the towns," she said, noting reports of house-to-house searches, arrests and random shooting coming from both towns.
Communications were cut off and, for the first time, the military has become directly involved in quelling the uprising, much to the disappointment of opposition activists.
"They were hoping the army would not get involved," Amin reported. "They feel this is only the beginning of a very serious crackdown."
Yet one activist told Al Jazeera that some army officers have defected to fight with the people of Deraa against the regime.
Two members stepped down from the provincial council in the southern region of Deraa, which has the highest death toll in the country. The resignations came a day after two lawmakers and a religious leader from Deraa also turned their backs on Assad in disgust over the killings.
'Barrier of fear'
Meanwhile, Syrian intellectuals expressed their outrage over the violence, with a declaration on Monday signed by 102 writers and exiles from all the country's main sects.
The declaration called on Syrian intellectuals "who have not broken the barrier of fear to make a clear stand.
"We condemn the violent, oppressive practices of the Syrian regime against the protesters and mourn the martyrs of the uprising."
Signatories included Alawite figures such as former political prisoner Loay Hussein; female writers Samar Yazbek and Hala Mohammad; Souad Jarrous, correspondent for the pan-Arab daily al-Sharq al-Awsat; writer and former political prisoner Yassin al-Haj Saleh and filmmaker Mohammad Ali al-Attassi.
Mansour al-Ali, a prominent Alawite figure from the city of Homs, was arrested in his home city after he spoke out against the shooting of protesters, another activist in Homs said.
At least 352 people have been killed in Syria since protests began, according to figures compiled by AFP.
And Wissam Tarif, executive director of INSAN, a Syrian human rights group, said that according to the organisation's most recent count on Friday there were 221 "forcefully disappeared people" in Syria.
The top United Nations human rights official called on Syria on Monday to rein in its security forces and investigate nearly 100 killings of protesters reported over the weekend.
Navi Pillay, the UN High Commissioner for Human Rights denounced the escalation of violence in the country and called for detained activists and political prisoners to be released.
"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," she said in a statement.