The MV Alaed, which is carrying helicopter gunships and air defence systems on board, will put to sea within days, military officials in Moscow were quoted as saying.

The vessel, sailing under the flag of the Caribbean island of Curacao, returned to Murmansk after its insurance cover was withdrawn at the behest of the Government. Russia said it feared that British forces would attempt to storm the ship, a step David Cameron is said to have contemplated.

Throwing down an unmistakable challenge to the Prime Minister, Moscow said the Alaed would sail again, this time under a Russian flag – meaning that any attempt to board it could trigger an international incident. It will also be accompanied by at least one other vessel, although not a military one.

Russia says the combat helicopters on board were ones that it had repaired and insists that it is only supplying Syria with weapons that can be used to defend itself from foreign aggression.

The efficiency of Russian-made air defence systems was potently displayed on Friday, when Syria shot down a Turkish fighter jet. The example could serve to deter the West from risking its own aircraft in a military intervention, analysts said.

Calls for international action have grown as the violence in Syria has worsened. Nearly 700 people were killed last week, according to pro-opposition human rights groups in Syria, making it one of the bloodiest since the uprising against President Bashar al-Assad began 15 months ago. More than 60 are said to have died on Sunday.

More than 150 government soldiers are thought to be among the dead. In a sign that morale in the armed forces is suffering, the Syrian opposition reported the defection of more than 30 soldiers after a rebel attack on an artillery battalion near the city of Aleppo. The defectors took their weapons with them, they said.

Last week, a pilot in the Syrian Air Force — considered the wing of the armed forces most loyal to Mr Assad — defected after flying his fighter jet to Jordan.