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As Syrians in government-held areas lined up outside polling stations to vote in the Presidential elections against the backdrop of the civil war, fighting continued in other areas of the country, with rebels likely preparing the next tunnel bomb.

Opposition fighters have made a growing use of underground tunnels to take the fight to the regime of Bashar al-Assad, who is widely expected to win a third seven-year term in office in what has been described by western powers as a sham vote.

A huge explosion rocked the Syrian city of Aleppo over the weekend, killing at least 20 government troops.

A massive cloud of dust was thrown into the air and, moments later, a shower of debris rained onto buildings' rooftops in the historic city.

The Islamic Front rebel coalition claimed responsibility for the attack which, they said, was carried out by digging under an army position and blowing it up with an underground bomb.

The tunnel bomb was the third to be detonated within a few weeks by Islamic Front units.

The group, which was formed last year with the reported backing of Saudi Arabia, has specialised in a technique that was widely used in the First World War's trenches.

Last year, The Times reported that Syrian rebels were being trained in tunnel building by Hamas military wing, the al-Qassam Brigades, which have extensive experience in building underground corridors to smuggle goods from Egypt to the Gaza strip.

Syrian rebels are instead tunnelling underneath enemy positions for surprise attacks to try and break the stalemate in fighting against government forces.

Earlier in May Islamic Front fighters in the northwestern Idlib province dug 850 metres under the Wadi al-Deif military base, which had been under siege by rebels for three years.

They set off some 60 tonnes of explosives, opening the way for the base to be finally captured, a rebel commander said.

Days earlier, the group used a tunnel bomb to destroy Aleppo's Carlton Citadel Hotel, which was being used by the army as a barracks, killing about 50 soldiers.

"Our holy warriors this morning blew up the Carlton Hotel barracks in old Aleppo and a number of adjoining buildings," the Islamic Front said announcing the operation on Twitter.

Aleppo, Syria's largest city, has been split into rebel and government held areas since anti-Assad forces launched a large offensive in mid-2012.

Tunnel bombs are also being used as a powerful propaganda tool by rebels who have otherwise suffered setbacks in some areas of the country.

The Islamic Front has posted spectacular online videos of all its recent attacks. After each explosions rebels are heard shouting "Allahu akbar" or "God is Great".