An Islamist militant group fighting President Bashar al-Assad's forces in Syria has said in a video posted online it has captured five Yemeni army officers sent by their government to help quell the Syrian uprising.

The video by Al Nusra Front showed clips of five men in civilian clothes asking the Yemeni government to stop supporting the Assad regime.

The authenticity of the recording could not immediately be verified.

One of the men identified himself as Mohammed Abdo Hezam al-Meleiky and said the Yemeni government had sent him and his colleagues to Damascus to help Assad's forces in the civil war raging across the country.

"I ask the Yemeni government to cut all logistical and military ties because Bashar al-Assad's regime is a regime that is killing its people and that is what we saw with our own eyes when we came here," he said in the online video.

The men's identity cards were shown in the online video, along with pictures of them in military uniform.

The Al Nursa Front has previously claimed attacks on Syrian government targets, including suicide attacks on the Syrian army staff building last week.

A Yemeni rights group said the five men were army officers studying at a military academy in Aleppo who went missing in August after fighting between rebels seeking to oust Assad and his opponents spread to the city in northern Syria.

The Hood group said five Yemeni families on 4 September had reported their sons had disappeared while heading from Aleppo to Damascus on their way home after completing their studies.

The five men it named included Meleiky. A sixth Yemeni, a doctor, went missing after he was stopped by Syrian authorities at Damascus airport on 13 August, it said.

"Hood had asked the foreign minister to exert all the efforts of the Yemeni government with the Syrian government and neighbouring governments to ensure the return of these citizens home," the group said in a statement sent to Reuters.

The 18-month-old uprising against Assad began as peaceful protests but has descended into a civil war.

More than 30,000 people have been killed, say activists.

Syria's government says it is fighting Islamist hardliners and thousands of Arab and foreign fighters have entered the country from Turkey.

Ancient markets burn as battle rages in Aleppo

A large fire in Aleppo destroyed parts of the city's medieval marketplace as rebels and government forces seek to gain control of the Syria’s largest city.

Unesco recognises Aleppo's Old City as a world heritage site and the organisation has described the damage as a tragedy.

The fire is believed to have been caused by the shelling and gunfire that began on Friday.

Archbishop of Aleppo Gregorios Ibrahim described the situation in the city as desperate.

The markets are a maze of vaulted passageways with shops that sell everything from foods, fabrics, perfumes, spices and artisan souvenirs.

They lie beneath Aleppo's fort where activists say regime troops and snipers have taken up positions.

Many of the shops have wooden doors, and clothes, fabrics and leather inside helped to spread the fire.

Rebels fighting to topple Syrian President Bashar al-Assad launched on Thursday what they said would be a "decisive battle" to drive his forces out of Aleppo and fighting has since spread to most areas of the city.

UNESCO believes five of Syria's six heritage sites, which also include the ancient desert city of Palmyra, the Crac des Chevaliers crusader fortress and parts of old Damascus, have been affected by the 18-month conflict.

Elsewhere, an advisor to Iran's Supreme Leader Ayatollah Ali Khamenei said he was confident Syria's government would beat the rebels.

"The victory of the government of Syria against internal opponents, America, and their other Western and Arab supporters, is counted as a victory of the Islamic Republic of Iran," said Ali Akbar Velayati.

"The victory of the Syrian government is certain."