David Cameron insisted that Nato's choice of targets in Libya was "in line" with the United Nations Security Council resolution amid claims that Muammar Gaddafi's youngest son had been killed in an attack by the coalition.
Saif al-Arab Gaddafi, 29, died when his house was hit by at least one missile fired by a Nato warplane, according to spokesman Moussa Ibrahim.
The Libyan government has accused Nato forces of breaking international law with the attack on the building in Tripoli, which is also said to have claimed the lives of three of the dictator's grandchildren.
The Prime Minister 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 said: "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."
The Prime Minister added: "Let's remember, while Gaddafi said that he wanted a ceasefire he was mining the harbour in Misratah in order to blow up vessels that were bringing humanitarian aid to help the people that he is murdering and killing with his snipers and rockets and artillery. We have got to remember that."
The Libyan regime spokesman said the air strike was a clear attempt to kill Colonel Gaddafi and was against "any moral code or principle".
In a press conference, Mr Ibrahim said: "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.
"If people claim they want to protect civilians we have again and again declared that we are ready for negotiation, ready for road maps for peace, ready for political transitional periods, ready for elections, ready for referendum.
"Nato does not care to test our promises. The West does not care to test our statements. They only care to rob us of our freedom, our wealth, which is oil, and our right to decide our future as Libyans."
Saif al-Arab Gaddafi was the sixth son of Gaddafi and brother of the better-known Saif al-Islam Gaddafi.
The younger Gaddafi had spent much of his time in Germany in recent years and had studied at a German university.
He "was playing and talking with his father and mother and his nieces and nephews and other visitors when he was attacked for no crimes committed", Mr Ibrahim said.
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 today its forces carried out precision strikes against Gaddafi regime military buildings in Tripoli, including a strike on "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."
Following the clams that the building attacked was in a residential area, Foreign Office minister Alistair Burt said it would be "naive" to assume that command and control centres would be identified as such by the Libyan military.
He told Sky News' Murnaghan programme: "There are military targets but individuals are not specifically targeted, that's always been the case and that remains the case."
He added: "Military targets are the ones which are sought out by the coalition forces.
"But it's naive to imagine that every command and control centre used by the Libyan regime will necessarily be labelled a command and control structure and be located in an army compound.
"Great care and effort is made to ensure that all the targets fall within the UN resolution but that targets may be disguised by others is not uncommon."Reuse content