Theresa May has failed to reassure MPs that "partially manufactured" British bombs are not being dropped on Yemeni civilians by the Saudi Arabian air force.

The Conservative premier was pressed by deputy SNP leader Angus Robertson on the issue at prime minister's questions (PMQs) today (19 October).

Robertson's grilling of May came after Amnesty International claimed the UK was "fuelling" the deadly conflict with arms sales to the Middle Eastern ally.

The human rights group apparently found evidence UK-made BL755 cluster bombs had been released on a village in Northern Yemen.

The weapon is designed to be released from UK-made Tornado fighter jets, some of which have been sold to Saudi Arabia under the Al-Yamamah arms deal.

"The Saudi air force is 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," Robertson said.

"I asked her a direct question and she didn't answer it so I can try again for a second time.

"Can she 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?

"If she doesn't know the answer to that question, how can she possibly in good conscious continue selling them to Saudi Arabia?"

May refused to be drawn on the question, instead calling for a probe into the allegations.

"We press for proper investigations into what has happened in those incidents before we reach a decision or conclusion on what has happened in those incidents," the prime minister said.

British Foreign Secretary Boris Johnson and US Secretary of State John Kerry called for a ceasefire in Yemen during a London press conference on Sunday.

"The conflict in Yemen is causing increasing international concern," Johnson said. "The fatalities that we have seen there have become unacceptable. There should be a ceasefire and that the UN should lead the way in calling for that."

The UN later announced that a 72-hour-long truce between the Yemeni government and rebel Houthi fighters had been agreed to. The ceasefire comes into force today.