BEIRUT,  - Nearly 50 people were killed on Sunday in violence across Syria, including 34 civilians who died in an assault in the restive central Hama province, the Syrian Observatory for Human Rights said.

Syrian regime forces rained shells on the village of Souran in Hama, it said, describing the deaths as a "massacre" and urging UN truce observers to deploy immediately in the area.

"Thirty-four people were killed under shelling and gunfire in Souran village while it was being raided," the Britain-based watchdog said, revising up an earlier toll of 16 people killed, including three children. "Where are the international monitors," read a statement issued by the Observatory.

"We in the Observatory express our extreme shock at the international monitors' failure to go to Souran when we issued our first statement on the killing 16 people," the statement added.

Elsewhere in the country another 14 people were killed in violence, the watchdog said, adding that anti-regime demonstrations were also held in several areas of Syria on Sunday.

The dead included a civilian killed by gunfire from regime forces who raided the village of Hasraya in Hama, one killed in Homs and two others in northern Aleppo, the Observatory said.

Meanwhile, a powerful blast went off within a few metres of a team of UN truce monitors visiting the Damascus suburb of Douma on Sunday, an AFP correspondent reported.

Regular army troops at the scene said the explosion was caused by a rocket-propelled grenade. The UN observers did not comment on the nature of the explosion, but said it was "imperative that all parties" stop fighting. "I think this is clearly one of these situations where it is absolutely imperative that all parties exercise restraint and do not engage in any more fighting," said UN peacekeeping chief Herve Ladsous.

He added that the UN monitors would investigate the Douma explosion, "as they investigate all the incidents that become known to them. They shall be reported." Both Ladsous and the head of the UN observer mission, General Robert Mood, were present at the site of the blast, which caused no injuries, an AFP correspondent reported.

The explosion took place a dozen metres away from where the UN team stood in Douma, a northern suburb of the Syrian capital, as violent clashes between rebels and regime troops were underway, the journalist said. The Syrian Observatory for Human Rights said earlier on Sunday that regime troops had launched rockets both near and into Douma.

Meanwhile, the United Nations said on Sunday that 480 Palestinian refugees have fled Syria to Jordan since the start of a revolt against President Bashar al-Assad's regime last year. "The UN Relief and Works Agency have registered 480 Palestinian refugees who have sought refuge in Jordan since the beginning of the crisis in Syria," UNRWA spokeswoman, Anwar Abu Sakina, told AFP.

"We know that there are more Palestinian refugees from Syria. Depending on their situation, some of the registered refugees get a one-time payment of between $300 to $900, while others receive cash and in-kind aid every three months."

Abu Sakina added that "half of those refugees live now in the northern governorate of Irbid." "There are 116 people in a guesthouse near the border town of Ramtha. The rest are in Amman and Zarqa, northeast of the capital," she said. "To my knowledge, only those who illegally entered Jordan from Syria are kept in the guesthouse."

UNRWA cares for about five million Palestinian refugees in the Middle East, more than two million of them in Jordan and around 475,000 in Syria.

According to the office of the UN High Commissioner for Refugees (UNHCR), 15,999 Syrian refugees are registered in Jordan.

But Amman says more than 100,000 Syrians have sought refuge in the kingdom since the March 2011 outbreak of the uprising, in which monitors say more than 12,000 people, the majority of them civilians, have died.