Heavy regime bombardment has damaged parts of the ancient citadel in Syria's world heritage site of Palmyra, an archaeological expert and an activist said on Thursday.

At least 13 barrel bombs have exploded on the citadel and its surroundings since Monday, according to Cheikhmous Ali of the Association for the Protection of Syrian Archeology, which monitors damage to the country's heritage sites.

Russia's defense ministry on Thursday said it will hold naval drills in the "east Mediterranean" in September and October, as the West frets over a military buildup by Moscow in Syria.

The exercises include three warships from Russia's Black Sea Fleet, including the Saratov landing ship, the Moskva guided missile cruiser and the Smetlivy destroyer, the ministry said in a statement.

Syrian President Bashar Assad attended holiday prayers at a Damascus mosque on Thursday, state media reported, in a rare public appearance for the embattled regime head.

Assad led Eid al-Adha prayers at the al-Adel mosque with state and ruling Baath party officials as well as a number of Muslim religious leaders and civilians, Syria's official news agency SANA reported.

German Chancellor Angela Merkel said Thursday that President Bashar al-Assad should be involved in any Syrian peace talks, amid mounting pressure to end the four-year conflict.

"We have to speak with many actors, this includes Assad, but others as well," Merkel told a press conference after an EU summit on the migration crisis sparked by the Syrian war.

Like tens of thousands of their fellow countrymen fleeing war, members of a Syrian rock group exiled in Lebanon undertook a perilous journey to reach Europe.

But their trek, which has already taken them through five countries, has led to an unexpected 'exile concert tour', on the way to their hoped-for final destination of Germany.

French President Francois Hollande on Wednesday called for a new Syria peace conference as part of efforts to end a conflict which has claimed some 250,000 lives and destabilized the whole region. 

"I call for a new peace conference so that all the countries who want to see peace restored in Syria can contribute," Hollande said.

The Syrian military for the first time on Wednesday deployed drones supplied by Russia for its fight against jihadists, a security source in Damascus said.

"For the first time, the army today used drones received from Moscow in operations against extremists in the north and east of the country," the source said, without elaborating on the type of drones or locating exactly where they were deployed.

U.S. military officials on Wednesday denied reports that the latest batch of American-trained Syrian rebels had defected to an al-Qaida affiliate.

The Pentagon is under intense scrutiny over its "train-and-equip" program in which moderate Syrian rebels are given instruction and weapons to fight Islamic State jihadists in the war-torn country.

The Islamic State group has released in a prisoner exchange one of two Kurdish journalists captured late last year, an official with Iraq's Kurdish television Rudaw said Wednesday.

Freelance reporters Massoud Aqeel and Farhad Hamo were on assignment for Rudaw in northeastern Syria when they were seized on December 15.

The Syrian regime has intensified its air strikes on the jihadist-held ancient city of Palmyra, killing more than 100 people including civilians in recent days, a monitoring group said Wednesday.

The fierce raids follow the delivery by Russia of new arms including warplanes to its ally Syria.