David Cameron has defended Nato's choice of targets in Libya after an airstrike apparently hit a building where Muammar Gaddafi was staying and killed his youngest son.

The Prime Minister insisted individuals were not being targeted, and UN Security Council resolutions permitted attacks on "command and control" bases.

But the Libyan regime reacted furiously, branding the missile strike an illegal assassination attempt.

Mobs also stormed diplomatic premises belonging to the UK and other Nato members in Tripoli, prompting Foreign Secretary William Hague to expel Libya's ambassador from London.

Saif al-Arab Gaddafi, 29, the dictator's sixth son, died when his house was hit by at least one missile fired by a Nato warplane, according to Libyan government spokesman Moussa Ibrahim. "This was a direct operation to assassinate the leader of this country. This is not permitted by international law. It is not permitted by any moral code or principle," Mr Ibrahim added.

Journalists taken to the walled complex of one-storey buildings in a residential Tripoli neighbourhood saw heavy bomb damage. The blast had torn down the ceiling of one building and left a huge pile of rubble and twisted metal on the ground.

Nato said its forces carried out precision strikes against Gaddafi regime military buildings in Tripoli, including a strike on a "known command and control building" in the Bab al-Azizya neighbourhood. But it did not verify media reports that members of Gaddafi's family had been killed.

Lieutenant-General Charles Bouchard, commander of Nato's Operation Unified Protector, said: "All Nato's targets are military in nature and have been clearly linked to the Gaddafi regime's systematic attacks on the Libyan population and populated areas. We do not target individuals."

Mr Cameron refused to comment on the "unconfirmed report" but told BBC1's Andrew Marr Show: "The targeting policy of Nato and the alliance is absolutely clear. It is in line with UN resolution 1973 and it is about preventing a loss of civilian life by targeting Gaddafi's war-making machine. That is obviously tanks and guns and rocket launchers but also command and control as well."

Mr Cameron added: "It is about targeting command and control rather than particular individuals. The targeting policy has been very closely followed, these things are very carefully put together."