At least 20 people, mostly women and children, were killed on Wednesday when United States-led coalition forces in Afghanistan carried out a cross-border attack in Pakistan's restive tribal belt, prompting Islamabad [Images] to condemn it as a 'provocation and gross violation' of its territory.

Witnesses said the attack occurred before dawn, when three or four helicopters of the North Atlantic Treaty Organisation's forces landed in Angoor Adda area of South Waziristan tribal agency.

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US and Afghan troops emerged from the helicopters, searched several houses and shot dead 20 people, witnesses and officials told TV channels.

Previous attacks by US-led forces on the Pakistani side of the border were carried out by pilotless drones armed with missiles. The use of ground troops prompted speculation that the strike was aimed at an important terrorist target.

Such actions would be counter-productive for joint efforts to fight terrorism, the Foreign Ministry said and described it as a 'gross violation of Pakistan's territory'.

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Among those killed in the strike were nine members of the family of a local tribesman named Taj Muhammad, whose home was searched by the troops, Geo News channel reported.

Reports suggested that at least five women and four children were among the dead.

Local tribesmen protested against the attack and shouted anti-America slogans.

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North West Frontier Province Governor Owais Ahmed Ghani, who is the administrative head of the tribal areas, condemned the attack as a direct assault on Pakistan's sovereignty.

A statement issued by the Governor said 'gunship helicopters and commandos' of the US-led coalition forces were responsible for the attack.

In a separate statement, the Foreign Office spokesman said the government condemned "the assault by coalition and International Security Assistance Force troops that has resulted in immense loss of civilian life".

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"This helicopter-borne ground attack, supported by air assets based in Afghanistan, is a gross violation of Pakistan's territory," he said.

"It is indeed most unfortunate that coalition and ISAF in Afghanistan have resorted to cross-border use of force against civilians.

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"Such actions are counter-productive and certainly do not help our joint efforts to fight terrorism. On the contrary, they undermine the very basis of cooperation and may fuel the fire of hatred and violence that we are trying to extinguish. Moreover, any attack on Pakistani territory is unacceptable and constitutes a grave provocation."

The spokesman said a "strong diplomatic protest is being made by the Foreign Ministry".

Prime Minister Yousuf Raza Gilani [Images] too condemned the attack, saying no external forces would be allowed to launch attacks in Pakistani territory.

Pakistan is a sovereign country "fully capable of countering extremist and terrorist elements within its borders on its own," he said.