BEIRUT – The United States proposed an international coalition to support Syria’s opposition Sunday after Russia and China blocked a U.N. attempt to end nearly 11 months of bloodshed, raising fears that violence will escalate.

Rebel soldiers said force was now the only way to oust President Bashar Assad, while the regime vowed to press its military crackdown.

The threat of both sides turning to greater force after Russia and China vetoed a U. N. Security Council resolution raises the potential for Syria’s turmoil to move into even a more dangerous new phase that could degenerate into outright civil war.

The uprising inspired by other Arab Spring revolts began in March with peaceful protests against Assad’s regime, sparking a fierce crackdown by government forces.

Soldiers who defected to join the uprising later began to protect protesters from attacks. In recent months, the rebel soldiers, known as the Free Syrian Army, have grown bolder, attacking regime troops and trying to establish control in pro-opposition areas.

That has brought a heavier government response. More than 5,400 people have been killed since March, according to the U.N., and now regime opponents fear that Assad will be emboldened by the feeling that he is protected by his top ally Moscow and unleash even greater violence to crush protesters. If the opposition turns overtly to armed resistance, the result could be a dramatic increase in bloodshed.

At least 30 civilians were killed Sunday, including five children and a woman who was hit by a bullet while standing on her balcony as troops fired on protesters in a Damascus suburb, according to the Britain-based Syrian Observatory for Human Rights, an activist group.

Government forces firing mortars and heavy machine guns also battered the mountain town of Zabadani, north of Damascus, a significant opposition stronghold that fell under rebel control late last month. Bombardment the last two days has wounded dozens and forced scores of families to flee, an activist in the town said.

“The situation is terrifying. Makeshift hospitals are full,” said the activist, who gave only his first name, Fares, for fear of government reprisal. He said the town has been under siege for the last five days, and there is a shortage of food and heating fuel.

The commander of the Free Syrian Army told The Associated Press that, after the vetoes at the U.N., “there is no other road” except military action to topple Assad.

“We consider that Syria is occupied by a criminal gang, and we must liberate the country from this gang,” Col. Riad al-Asaad said, speaking by telephone from Turkey. “This regime does not understand the language of politics. It only understands the language of force.”