An air raid on Syria's famed Crac des Chevaliers castle, a UNESCO World Heritage site, has damaged one of the fortress's towers, footage shot by activists showed.

Several videos posted online on Saturday showed at least one air strike on Friday against the Crusader castle in central Homs province, where fighting is raging between government troops and rebel forces.

The footage shows a huge blast as a tower of the fortress, which is built on a hill, appears to take a direct hit, throwing up large clouds of smoke and scattering debris in the air.

A separate video filmed inside the castle purports to show some of the damage caused by the air strike, including a gaping hole in the ceiling and a pile of rubble below.

"God is great. This is the destruction caused by MiG air strike on the Crac des Chevaliers," says the activist filming the damage.

The Syrian Observatory for Human Rights, an activist group, could not confirm direct hits on the castle, but said there were reports of three air strikes in the area on Friday.

The raids came after rebels apparently using the Crac des Chevaliers as a base attacked an Alawite village called Qumayri, killing several people, Observatory director Rami Abdel Rahman told the AFP news agency.

The Crac des Chevaliers was built between 1142 and 1271, according to UNESCO, and along with the Qalat Salah el-Din fortress in Latakia province, is considered one of the best preserved Crusader castles in existence.

Earlier this year, UNESCO decided to add all six of Syria's World Heritage sites to its World Heritage in Danger list, reflecting concerns that serious damage was being inflicted on the areas as the country's conflict continues.

Homs besieged

Fighting in and around the Crac des Chevaliers has been reported throughout the conflict, which began with anti-government protests in March 2011 and evolved into an armed uprising after crackdowns on demonstrations.

Rebel-held areas of Homs city have been under intense assault from government forces since June 28, and UN officials said in a joint statement on Friday that they are "extremely alarmed by the escalating violence" in Homs and Aleppo.

UN humanitarian chief Valerie Amos and human rights chief Navi Pillay renewed appeals for access to the city but said Assad's forces and opposition rebels were refusing to give firm safety guarantees.

The UN and the International Committee of the Red Cross said they were ready to rush in aid if they could secure a halt to the fighting.

"We understand that up to 2,500 people remain trapped inside Homs, where there are reports of continuing shelling, use of long-range weapons and ground attacks using tanks," the two officials said.

"The presence of armed opposition groups inside residential areas also increases the risk for civilians," they added in an appeal to the warring sides to "grant immediate safe passage to allow civilians to leave Homs and to allow humanitarian aid to get in."