Rocket-propelled grenades struck the headquarters of Syria's ruling party Sunday, bringing the violence that has engulfed much of the country to the heart of its capital for the first time, activists said.

The attack on the building in Syria's capital of Damascus apparently caused no damage or casualties. But if true, it would mark a significant shift in the eight-month uprising against President Bashar Assad. Until now, the capital has remained relatively untouched.

The pre-dawn attack awoke many Syrians who reported hearing two loud blasts, but details could not be confirmed. The foreign minister denied an attack had taken place, and Syrian television broadcast footage of the building looking undamaged.

The Free Syrian Army, a group of military defectors, claimed responsibility, highlighting the growing militarization of the revolt following months of largely peaceful protests.

"This is an escalation that would signal a new phase in the Syrian uprising," said Thabet Salem, a Damascus-based analyst. "It gives a new dimension to the whole situation, which had been so far restricted to (government) action, and reaction from the opposition," he said.

Syria has placed severe restrictions on the work of journalists, making it extremely difficult to confirm events.

The country's embattled but defiant president vowed to pursue his bloody crackdown on dissent, saying it was his "duty" as president to crush the militants he blames for the unrest.

Syria's uprising against Assad, although largely peaceful, has grown more violent in recent weeks as frustrated protesters realize the limits of peaceful action. Army dissidents who sided with the protests have also grown bolder, fighting back against regime forces and even attacking military bases and raising fears of a civil war.

Foreign Minister Walid al-Moallem dismissed the idea of civil war, saying such talk was "wishful thinking" by the West.

The British-based Syrian Observatory for Human Rights said unknown gunmen on motorcycles threw first a stun grenade and then fired RPGs at the Baath party headquarters, hitting the outside wall of the building. Two other grenades missed the target, it said.

Omar Idilbi, spokesman of the Local Coordination Committees' activist network, also said the perpetrators were masked men on motorcycles. But he cast doubt on the claim of responsibility by the Free Syrian Army, saying the target was suspicious.

"The Baath party headquarters is not a military or security base to be attacked by the FSA," he said.

The foreign minister said reports of an attack were "absolutely baseless." But at a news conference, he thanked a Syrian journalist for clarifying that the explosions heard in the area were the result of a stun grenade, indirectly acknowledging that some kind of attack had taken place.

In Cairo, the Arab League rejected amendments proposed by Syria to a peace plan to end the crisis, saying the changes alter the plan's "essence." The rejection is a further blow to Syria, a country that prides itself on being a bastion of Arab nationalism.

Last week the body reaffirmed its suspension of Syria and gave it three days to comply with the plan, which calls for the withdrawal of government tanks from the streets, the release of political prisoners and a halt to attacks on civilians.

The 22-member organization did not give details of Syria's proposed amendments. It said in a statement Sunday that Damascus' proposals were unacceptable because they introduce "drastic changes" to the mandate of a proposed league mission to ensure the plan is implemented.

Al-Moallem told reporters the proposed observers' mission included "impossible conditions" and gave excessive authority to the observers in a way that violated Syria's sovereignty. He denied Damascus sought to restrict the observers' movement within the country but said Syria wanted to be informed of the groups' travels in order to offer security protection.

"The observers, if they come, will have freedom of movement," he said. "We have nothing to hide. They should see the killings, the massacres and the crimes being carried out against our people and security forces."

An Arab League official, speaking on condition of anonymity because he was not authorized to brief reporters, said the Syrian government was required to implement the peace plan in its entirety.

In an interview with Britain's Sunday Times newspaper, Assad vowed to crush "militants" who he says are massacring Syrians.

"The role of the government is to fight those militants in order to restore stability and to protect civilians," he said, repeating warnings that foreign military intervention in Syria would "shake the entire Middle East."

On Sunday, activist groups said at least nine people were killed by security forces, including five in the flashpoint central city of Homs and four in northern Syria.

The U.N. says more than 3,500 people in Syria have been killed in the crackdown since the start of the uprising in mid-March. The LCC network on Sunday said at least 280 children had been killed in the crackdown.

Assad, in the interview, said more than 800 Syrian officers and security forces had been killed.

In the interview, Assad said he feels "pain and sorrow" for the bloodshed but insisted the solution was to eliminate the militants he blamed for much of the violence. The Assad regime maintains the militants are following a foreign agenda to isolate and weaken Syria.

Assad, who took over from his late father, Hafez, in 2000, said there would be parliamentary elections in February or March, after which there would be a new government and new constitution.

"That constitution will set the basis of how to elect a president. ... The ballot box should decide who should be president."