The SNP has called on David Cameron to "admit" that Britain is "effectively taking part in a war" in Yemen that is costing thousands of civilian lives.

Speaking during Prime Minister's Questions, the party's Westminster Group Leader Angus Robertson claimed Mr Cameron had "not sought Parliamentary approval to do this".

Mr Cameron hit back, arguing Mr Robertson had "started in a serious place but then seriously wandered off".

The MP for Moray said: "Thousands of civilians have been killed in Yemen, including a large number by the Saudi air force, and they've done that using British-built planes with pilots who are trained by British instructors who are dropping British-made bombs and are co-ordinated by the Saudis in the presence of British military advisers.

"Isn't it time for the Prime Minister to admit that Britain is effectively taking part in a war in Yemen that is costing thousands of civilian lives and he has not sought Parliamentary approval to do this."

Mr Cameron said it was in Britain's interests that "we back the legitimate government of Yemen and it's right to do that".

He added: "We have some of the most stringent arms control measures of any country anywhere in the world. But just to be absolutely clear about our role, we're not a member of the Saudi-led coalition, British military personnel are not directly involved in the Saudi-led coalition's operations.

"Personnel are not involved in carrying out strikes, directing or conducting operations in Yemen or selecting targets and we're not involved in the Saudi targeting decision-making process.

"But yes, do we provide training and advice and help in order to make sure that countries actually do obey the norms of humanitarian law, yes we do."

Mr Robertson used both of his party's two questions during PMQs to press Mr Cameron on the issue of Yemen, asking: "Can the Prime Minister tell the House what the UK Government is doing to support peace in Yemen?"

Mr Cameron said: "Well, we are doing everything we can with all the people taking part in this conflict to encourage them to get round a negotiating table as they have done recently in order to bring about what's necessary in Yemen, which is a government that can represent all of the people.

"You've got to make sure that both Sunni and Shia are properly represented in that country and that's the only way that we'll meet our key national interest, which is to back a government in Yemen that will drive the terrorists including al Qaida in the Arabian Peninsula (AQAP) out of Yemen, because they have been and are a direct threat to the citizens of Britain.

The Prime Minister's official spokeswoman sought to differentiate between British staff based in the Saudi-led coalition control centres "in a liaison capacity only" and ongoing training schemes.

A "very small number" of personnel were being used to "help provide insight for us into the operations ... so that we can make informed assessments of the actions of the coalition" in terms of complying with international law and other issues, she told reporters.

"They are not involved in the targeting process, whether that is selection, decision making or directing."

She declined to give details of what feedback had been received.

Separately, the UK provides training on targeting - including on UK-based courses - to the Saudi military, she said.

"That is a long-standing arrangement and it will continue".