Syria's government said Tuesday it was ready to act on a United Nations call to evacuate civilians trapped in the rebellious central city of Homs for more than a week, but blamed rebels for obstructing efforts to get them out.

Maj. Gen. Robert Mood, chief of the UN observer mission in the country, has demanded that all warring parties in the conflict allow safe passage for women, children and sick people who need to leave the city and other combat zones.

Activists said shelling and clashes between rebel fighters and troops in Homs continued unabated Tuesday, underlining the difficulty in organizing any sort of evacuation. Fierce shelling was reported on the rebel-held districts of Khaldiyeh and Jouret el-Shayeh and nearby areas. Troops backed by helicopter gunships were also bombarding the town of Rastan, north of Homs, which is controlled by rebels.

The Britain-based Syrian Observatory for Human Rights reiterated an urgent appeal for world leaders to intervene to stop the shelling that has trapped more than 1,000 families and "the systematic killings that the Syrian people in Homs are being subjected to."

A Foreign Ministry statement carried by state-run news agency SANA said the government has contacted the UN observer mission and local authorities in Homs to start efforts to bring out the trapped civilians.

"But the efforts of the UN monitors' mission failed in achieving this goal because of the armed terrorist groups' obstructions," the statement said. It charged that armed groups were using innocent civilians as "human shields."

The Syrian government regularly refers to the rebels as terrorists.

Mood said Tuesday that UN forces have come under fire multiple times recently but are committed to staying in the strife-torn country.

After a private briefing of the UN Security Council he said questions about cancelling the mission were premature and noted, "We are not going anywhere."

The UN said Saturday its 300 observers based in Syria were suspending all missions because of concerns for their safety after fighting intensified over the previous 10 days.

"Shelling, small arms fire and other incidents are coming much closer, and we have been targets several times over the last few weeks," Mood said, adding it was not only dangerous to his observers, but made it difficult to carry out their mission.

A UN diplomat who was at the briefing, and who spoke on condition of anonymity because the briefing was closed, said Mood told the council that observers have suffered direct fire at least 10 times, have been in several indirect fire incidents, and nine UN vehicles were damaged or struck by small arms fire.

"The suffering of the Syrian people, the suffering of men, women and children, some of them trapped by fighting, is getting worse," Mood told reporters.

Mood did not attribute the suspension of the mission to any single incident, but said that among the problems was the inability of the observers to get through an angry crowd that mobbed their motorcade in one city.

Reviving the mission would require a reduction in violence and a commitment from both the Syrian government and the opposition to "freedom of movement" for the UN observers, Mood said.

While the Syrian government had given him such assurances in the last few days, he said "I have not seen the same clear statement from the opposition yet."

UN peacekeeping chief Herve Ladsous, who also briefed the council, added: "For the time being we have decided, however, not to touch, not to modify, but to rather retain the integrity of the mission and its mandate. We must be aware that a number of diplomatic initiatives are under way, bilateral and multilateral."

The diplomat who spoke on condition of anonymity said Mood told the council that the observers are trying to facilitate a local ceasefire in Homs.

The diplomat added that Mood told the council there has been little effort to ensure civilian protection by the Syrian government, and there has been no increase in the pace and scale of release of people who are arbitrarily detained.

Mood said that on a smaller scale the opposition is depriving liberty to individuals too, the diplomat said.

Mood spoke to the Security Council amid growing concern that the escalating violence may spell the end of the monitoring effort and international envoy Kofi Annan's peace plan.

British UN Ambassador Mark Lyall Grant had said prior to the briefing that council members were eager to hear from Mood.

"I think there will be a lot of member states of the council, including us, who will be questioning now what the future is for the mission and, therefore, by extension the Annan plan, in light of these recent developments on the ground," he said.

The three-month mandate of the observer mission expires July 20. Grant said he could not rule out that the council might end it before then.

Opposition groups say more than 14,000 people have been killed since the Syrian uprising began in March 2011.