Syrian forces fired tanks and machine-guns Wednesday in an attack on the southern city where the anti-government uprising began, propelled by recent military victories over rebels as the opposition fell into disarray.

At least two prominent Syrian dissidents said they have quit the main opposition group, the Syrian National Council, calling it an "autocratic" organization.

The opposition has been hobbled by disorganization and infighting since the popular revolt against President Bashar Assad began a year ago. Its international backers have repeatedly appealed for the movement to pull together and work as one unit.

Even before the high-level resignations, the opposition was already reeling from significant losses over the past few weeks when rebel fighters were largely driven out of two key strongholds -- the city of Idlib near the northern border with Turkey and the Baba Amr district in the central city of Homs.

After retaking Idlib earlier this week, government forces launched new attacks Wednesday on Daraa, the birthplace of the uprising.

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A Jordan-based spokesman for the rebel Free Syrian Army, Muneef al-Zaeem, said there were tanks in the area and reports of dead and wounded, but that numbers could not be confirmed.

"We are not able to leave our houses. People are panicking because of the shooting," Daraa resident Abu Ahmed told The Associated Press by phone. "We are fearful because the shells can explode and kill everybody present in our homes."

The series of government assaults on opposition strongholds has only intensified international pressure on Assad. The U.S. had said it was considering military options to aid the Syrian rebels. But President Obama said Wednesday that military intervention in the conflict would be premature, could lead to civil war and more deaths.

French President Nicolas Sarkozy likened Assad to a "murderer."

However Russia, which has remained a steadfast ally to Assad, said it would continue arming his regime -- reflecting the ongoing divisions in the international community over how to bring the violence in Syria to an end.

Although most countries are unwilling to get involved in the conflict, for fear of a protracted battle and a possible civil war, there have been widespread calls for the opposition to unite and speak with one voice against Assad.

Indeed, Assad's greatest advantage has been the weakness and lack of unity among the disparate forces opposing him. The groups have various ideologies and motivations, from secular forces to religious conservatives to outright radicals.

On Wednesday, the opposition's disarray was on display.

Kamal al-Labwani, one of the dissidents who announced his resignation from the Syrian National Council, said the 270-member organization was run autocratically. He even compared it to Assad's ruling Baath party.

He blasted council chief Burhan Ghalioun and others in the SNC.

"They are trying to build an autocratic rule inside the council," said al-Labwani, who worked for years against the Assad family regime before being jailed in 2005. He joined the council soon after being released in November.

"There is no group work. Everyone is working by himself and the whole council has not met once," confirming outside observations about the lack of unity in the movement.

He said that another council member, Catherine al-Talli, also quit and said he expected many more to follow suit soon.

Neither Ghalioun or al-Talli could be reached for comment. Council spokeswoman Bassma Kodmani did not respond to an email requesting comment.

Another dissident, 80-year-old lawyer Haitham al-Maleh, also said he quit over lack of cooperation in the council.

"There is no transparency and there is no respect for other opinions," he said. "They aren't given the work the attention that it needs to fulfill the ambitions of the Syrian people."

Both men said they would remain involved in the quest to topple Assad. Al-Labwani called for an international conference to be held in Turkey to make the council more democratic while al-Maleh said he was working to build support for armed rebels of the Free Syrian Army.

The U.S. and many European and Arab countries have said Assad must go but remain opposed to military intervention. Joint U.N.-Arab League envoy Kofi Annan ended his first visit to Syria Sunday without obtaining a cease-fire to make way for political dialogue.

Also Wednesday, Amnesty International said Syrian security forces routinely torture people detained during the uprising. The London-based group said detainees are beaten with sticks, cords and rifle butts. Others are sexually assaulted or killed.

The torture appears to be part of a strategy to punish and intimidate dissidents, the group said in its report.

"Torture and other ill-treatment in Syria form part of a widespread and systematic attack against the civilian population, carried out in an organized manner and as part of state policy and therefore amount to crimes against humanity," according to the report.

Amnesty's report was based on interviews in mid-February with dozens of Syrians who had fled to neighboring Jordan. Twenty-five said they had been tortured or ill-treated.

The group said it has documented 276 cases of death in detention since the uprising's start. But given the large number of people who have been detained, it says the number of those killed is likely much higher.

The report also accuses armed opposition groups of kidnapping and killing people believed to be associated with the regime. Syrian officials were not immediately available for comment.