Three Pakistani soldiers were killed and another three injured when NATO helicopters shelled a Pakistani outpost in the restive tribal belt bordering Afghanistan on Thursday, in the fourth cross-border attack into Pakistan in less than a week.

“There are reports of missiles fired on our post in Kurram Agency, killing three soldiers. Our authorities are investigating (the incident),” Foreign Office spokesman Abdul Basit told a news briefing.

State-run PTV quoted security sources as saying that NATO helicopters crossed into the Kurram tribal region from Afghanistan and hit a Pakistani border post.

Official sources said three soldiers were injured when the helicopters hit the Mandato Kandao post early Thursday morning.

This was the fourth NATO air strike on Pakistani territory in a week. On September 26 and 27, NATO helicopters shelled Pakistani areas and officials claimed over 50 militants were killed in the air strikes.

A NATO spokesman in Afghanistan said the air strikes into Pakistan were carried out after militants attacked an Afghan security outpost near the border.

The NATO spokesman justified the rare air strikes by foreign forces in Pakistani territory as being based on “the right of self-defence”.

Pakistan strongly protested against the aerial engagements from Afghanistan, saying they were unacceptable and a violation of its sovereignty.

It said the mandate of NATO forces ended at the border. Islamabad also insisted that there were no agreed “hot pursuit” rules between Pakistan and U.S.-led forces in Afghanistan.

Mr. Basit said that as a result of Pakistan’s “strong demarche to NATO earlier this week, we were assured that ISAF and NATO would strictly abide by its mandate and not infringe on Pakistan’s sovereignty.”

Pakistan is determined to protect its sovereignty “under all circumstances”, he said.

Islamabad will formulate its response after completing its investigation into the incident in Kurram Agency, he said.

Mr. Basit evaded a question on options available to Pakistan if the attacks by NATO forces continued, saying the country would not allow its sovereignty to be violated.

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