Theresa May has refused to be drawn on whether missiles manufactured in the UK and sold to Saudi Arabia have been responsible for civilian deaths in Yemen.

The Government has faced fierce criticism for selling arms to the country for potential use in Yemen's bloody civil war which pits the Yemeni government, backed by Saudi Arabia, against Shiite Yemeni rebels.

Ministers have faced repeated calls to impose a ban on the sale of weapons amid concerns that international humanitarian law could be being broken in the conflict.

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Angus Robertson, the SNP's leader in Westminster, demanded at Prime Minister's Questions to know whether missiles partially built in Scotland had been responsible for the deaths of innocent civilians.

But Mrs May would not be drawn as she said the Government had "pressed" Saudi Arabia to "properly investigate" any incidents of concern.

Mr Robertson said: "Thousands of innocent civilians have now been killed by Saudi air strikes in Yemen.

"Can the Prime Minister give the House an assurance that those civilians have not been killed by Paveway IV missiles, partially manufactured in Scotland under licence from her Government to Saudi Arabia?"

Mrs May replied: "As you know we have one of the toughest regimes in the world in relation to arms exports.

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"When these allegations arise then we have been pressing, I have pressed in the past and the Foreign Secretary has pressed, the Saudi Arabian government to properly investigate these issues and to learn the lessons from them."

Mr Robertson asked his question again.

"It's beyond doubt that Saudi air forces bombing Yemen, flying planes that are made in Britain, by pilots that are trained by Britain, and they are dropping missiles that are made in Britain," he said.

"I asked you a direct question and you couldn't answer it, so I can try for a second time.

"Can you give this House an assurance that civilians have not been killed by Paveway IV bombs being dropped on Yemen which are partially manufactured in Scotland under licence by the Government, and if you don't know the answer to that question how can you possibly in good conscience continue selling them to Saudi Arabia?"

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Mrs May reiterated that the UK urges the Saudi government to conduct thorough investigations into any incidents "before we reach a decision or a conclusion".

She said: "We have a very strong relationship with Saudi Arabia - that is important for this country.

"It's important in terms of dealing with counter-terrorism and a number of other issues.

"But what matters when incidents happen about which there is concern is that they are properly investigated."