A rocket-propelled grenade exploded near a team of U.N. observers in a Damascus suburb on Sunday, the military said, as clashes between regime troops and armed rebels raged in and around the Syrian capital and at least 21 people were killed across the country.

No one was hurt in the Douma blast, which came as U.N. truce mission head Major General Robert Mood and peacekeeping chief Herve Ladsous were leading a team of observers around the north Damascus suburb, an Agence France Presse correspondent at the scene said.

The United Nations said on Sunday that 480 Palestinian refugees have fled Syria to Jordan since the start of a revolt against President Bashar Assad's regime last year.

"The U.N. 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.

Syrian troops killed a civilian and an army defector, a watchdog said, adding that anti-regime demonstrations were held in several areas of northwestern and central Syria on Sunday.

"A civilian was killed after he suffered gunshot wounds from regime troops, who were raiding Hasraya village" in the central province of Homs, the Britain-based Syrian Observatory for Human Right said.

Two high-ranking Syrian officials denied in interviews with state television on Sunday reports by pan-Arab channels that they along with four other regime stalwarts had been assassinated.

The interviews came after satellite channels Al-Jazeera and Al-Arabiya broadcast an amateur video showing an unidentified man claiming a rebel group's responsibility for killing the six officials.

Interior Minister Marwan Charbel stated that the situation in the northern city of Tripoli is improving “day after day”, questioning the motives of the United Arab Emirates, Bahrain, and Qatar’s travel restrictions to Lebanon, reported the pan-Arab daily Asharq al-Awsat on Sunday.

He told the daily An Nahar: “The restrictions are a precautionary measure ahead of the touristic summer season.”

Fierce fighting between regime troops and armed rebels rocked parts of the Syrian capital Damascus overnight, a monitoring group said on Sunday.

"Violent clashes broke out between rebel fighters and regime troops at a checkpoint in Kafr Sousa district," the Syrian Observatory for Human Rights said in a statement.

A Syrian was shot dead and two others wounded as they tried to cross the border from Lebanon back into their country Saturday, security and medical sources said.

The three men had left the northern Lebanese village of Muqaibleh and were trying to reach the nearby Syrian village of Msherfeh when they were shot at, the sources said.

Russia's non-intervention stance on Syria remained unchanged on Saturday as G8 leaders looked to hammer out a joint declaration to put greater pressure on President Bashar Assad's regime.

"There cannot be any change of regime through force," the Kremlin's Africa envoy, Mikhail Margelov said, adding that G8 leaders meeting at Camp David had yet to agree on the Syria part of their final summit declaration.

U.S. President Barack Obama said Saturday that the G8 -- which includes Russia -- agreed that the political process in Syria should move forward "in a more timely fashion."

"We had a discussion about Syria, we all believe that a peaceful resolution and a political transition in Syria is preferable," Obama said, flanked by leaders of the G8 industrialized nations at a Camp David summit.

Prime Minister Najib Miqati urged on Saturday the United Arab Emirates, Qatar and Bahrain to reconsider their travel warning to Lebanon.

The premier said "the procedures taken by some Gulf countries are not justified," urging "the three countries to reconsider the travel restriction advisory to Lebanon."