Syrian troops intensified their shelling of rebel-held neighbourhoods in central and northern Syria on Sunday, killing at least 50 people and wounding hundreds, hours after UN monitors suspended their work, activists said.

On Sunday night the head of the UN observers' mission in Syria, Major General Robert Mood, demanded that both sides allow the evacuation of endangered women, children, elderly and sick from the besieged city of Homs and other combat zones.

He said neither Syrian troops nor rebels were willing to hold their fire to allow them to leave, particularly from Homs, and UN efforts to evacuate civilians from Homs had so far failed.

Homs has been under siege for a week, part of an escalation of violence around Syria that forced the 300-strong UN observer mission in the country to suspend its activities on Saturday.

"The humanitarian situation in Homs is very difficult," said Rami Abdul-Rahman, head of the UK-based Syrian Observatory for Human Rights (SOHR). "It is very clear that the army wants to retake Homs."

"Around 85% of Homs is now under shelling or bombardment with mortar rounds and heavy machine guns," opposition campaigner Abu Imad told Reuters by phone from the hot spot of the 15-month revolt, about 80 miles north of the Syrian capital Damascus.

"Dozens of wounded are without treatment because all the hospitals have fallen under the control of shabbiha [ghosts]," he said, referring to militiamen loyal to Assad.

"The dead are the lucky ones."

Another opposition activist, Mohammad al-Homsi, said: "Since the [UN] observers stopped working yesterday we have seen a clear escalation."

The UN observers had been the only working part of Kofi Annan's peace plan, which the international community sees as its only hope to stop the bloodshed. His plan called for monitors to check compliance with a ceasefire that was supposed to take effect in April, but government and rebel forces have largely ignored the truce.

The statement suspending the mission reinforced fears that Syria was sliding ever closer to civil war, 15 months after the start of the uprising aimed at ousting Bashar al-Assad as president. Opposition groups say more than 14,000 civilians and rebels have been killed since March 2011.

In Turkey, the leader of Syria's main opposition group, Abdulbaset Sieda, said in a speech the suspension of observers' activities showed "the international community has given up hope on this regime that is in its last days". He said Assad's government had lost control over many large areas and was "now suffering from confusion and committing more crimes as revenge".

"The international community must bear its … responsibilities to take decisive decisions through the security council under chapter seven to protect civilians," he said. A chapter seven resolution authorises enforcement actions that can ultimately include military force.

The Syrian government has been waging an offensive in towns and villages nationwide for the past week, trying to flush out rebels by shelling urban areas with tanks and attack helicopters. Rebels have attacked Syrian forces, mostly trying to burn out their tanks.

The foreign secretary, William Hague, said the development underlined the need for the international community urgently to come together to compel the regime to meet its commitments. "The United Nations security council will be considering its options," he said in a statement.

Washington reiterated its call for the Assad regime to comply with the Annan plan, "including the full implementation of a ceasefire".

The Syrian government said it had told Mood that it understood the decision to suspend the observer mission and blamed the rebels for the escalation in fighting.

Both groups also reported intense clashes in the Damascus suburb of Mleiha, and violence in the northern provinces of Idlib and Aleppo, the eastern region of Deir el-Zour and the southern province of Deraa. The LCC said four people from both sides died in the fighting in Mleiha.

Rebels attacked an army checkpoint in central Hama province, killing at least three soldiers, the SOHR said. Both groups also reported violence in the northern provinces of Idlib and Aleppo as well as the eastern region of Deir el-Zour and the southern province of Daraa.

Syria's state-run Sana news agency said troops had fought infiltrators from Lebanon on Saturday, killing six and wounding four of them. It added that Syrian forces also foiled an infiltration attempt from Turkey into the northern province of Idlib.

Syrian authorities say weapons are being smuggled to rebels from neighbouring countries.