U.N. ceasefire monitors visited the Syrian city of Homs on Saturday after months of bombardment, hours before the U.N. Security Council was to vote on a draft resolution to authorise the deployment to Syria of up to 300 unarmed military observers.

Opposition activists in Homs, epicentre of the 13-month-old revolt against President Bashar al-Assad, said shelling and gunfire stopped for the first time in weeks before Syrian authorities let the monitors into the city.

A handful of monitors has been in the country for a week as an advance party while diplomats hammer out the mandate for a force of hundreds. During that time, a ceasefire has so far failed to end violence in the worst-hit parts of the country.

A massive explosion was heard near a military airport in the Syrian capital Damascus on Saturday, a resident said, adding that he was not sure what had caused the blast. No further details were immediately available.

A team of observers have been sent to Homs and met the governor, U.N. spokesman Khaled al-Masri said. They are now visiting districts of the city.

Amateur video footage posted on the internet showed the monitors, clad in turquoise bullet-proof vests, being escorted by hordes of opposition residents through rubble-filled streets.

The people want the overthrow of the president, chanted the residents, many carrying the Syrian revolutionary flag.

Activists said the shelling let up only to make it look as if the government was abiding by the truce, mediated by international peace envoy Kofi Annan, and they expected shelling would resume as soon as the monitors left.

Despite U.S. and European concerns that Damascus has yet to fully implement the ceasefire, the 15-nation Security Council plans to vote at 1100 EDT (1500 GMT) on a compromise resolution that combines Russian and European drafts.

U.N. action must be approved both by Western countries that want to see Assad toppled and by Russia and China, his veto-wielding allies on the Security Council, who have insisted that the observers' mandate be approved by Damascus.

Assad's opponents fear that a few hundred observers with a weak mandate would act as little more than a fig leaf for the government, thwarting more robust intervention to halt the bloody crackdown on cities that have risen up against Assad.

A similar-sized Arab League observer mission collapsed in failure in January after just a month. But Annan's staff have argued that a small observer force can still help improve the situation by changing the political conditions on the ground.

The wobbly ceasefire was further undermined on Friday when at least 42 people were killed - 15 of them by two roadside bombs targeting security forces and many of the others by government forces' shelling of Homs.

On Saturday, the Syrian state news agency said that an armed terrorist group had blown up an oil pipeline in the eastern province of Deir al-Zor, near the border with Iraq.

On Friday, U.N. envoys reached preliminary agreement on a draft resolution on the Syrian conflict, which has divided the Security Council since it erupted in March 2011. The deal could still collapse as diplomats need approval from their capitals.

It's possible not everybody will have instructions at that point (11 a.m.), U.S. Ambassador to the United Nations Susan Rice, president of the Security Council this month, told reporters on Friday. It's possible there will not be an agreed text at that point, we'll see, and we'll regroup accordingly.

Britain, France and Russia would also like a deal.

I hope there's going to be a unanimous vote, said Russian Ambassador Vitaly Churkin. The Syrian government and the opposition must know that the Security Council will be authorising the full-fledged mandate so we hope it's going to send a strong and good political signal.

Eight monitors are already in Syria from Morocco, Brazil, Belgium, Switzerland and Norway after the council authorised an advance team of up to 30 on Saturday. A spokesman for that team told Reuters two more monitors were due to arrive on Monday.

The new resolution is needed for a further initial deployment of up to 300 as recommended by U.N. Secretary-General Ban Ki-moon.

Some council members have expressed reluctance to give swift approval for an expanded observer mission because of concern about the Syrian government's failure to halt the violence.

The main opposition group, the Syrian National Council, has welcomed the observers' mission and said that more monitors must be deployed to serve as witnesses.

FEW SIGNS OF CEASEFIRE

The observers' task is to oversee the ceasefire mediated by Annan, a former U.N. secretary-general. The plan calls for an end to fighting by government security forces and rebels, withdrawal of heavy weapons from towns, the return of the army to barracks, humanitarian access and dialogue between the government and opposition aimed at a political transition.

Annan's deputy, former Palestinian foreign minister Nasser al-Kidwa, criticised both sides, but particularly government forces, for refusing to stop fighting completely.

We don't see much of a ceasefire, he told France 24 television. The situation of course is not good. There are many reasons to be worried by the lack of implementation, at least lack of full implementation by (the) Syrian government and perhaps some other parties as well.

Top U.N. humanitarian official John Ging said on Friday he hoped Syria would also grant permission in the coming days for more aid workers to enter the country, where at least 1 million people need of urgent assistance.

He told reporters in Geneva that Damascus had recognised there were serious humanitarian needs and that action was required, but logistical issues and aid workers' visas were still being discussed.

After shelling the central Homs district of Baba Amr for a month, the Syrian government prevented the International Committee of the Red Cross from entering the area for several days. Opposition activists living in Homs said the government wanted to erase evidence of war crimes.

On Thursday, Syria and the United Nations signed an agreement setting out the working conditions of ceasefire observers. It stipulates unfettered access and freedom for monitors to travel and make contact with people.

But Moroccan Colonel Ahmed Himmiche's advance team was last week denied permission to go to Homs by the Syrian authorities - for purported security reasons.

Separately, the U.N. refugee agency has appealed for $84 million for 60,000 Syrian refugees who have fled to Jordan, Lebanon, Turkey and Iraq. It has received only 19 percent of the funds.

(Additional reporting by Michelle Nichols at the United Nations, John Irish in Paris and Stephanie Nebehay in Geneva; Writing by Oliver Holmes; Editing by Peter Graff)