The Pakistani army says at least eight suspected al-Qaeda fighters and two of its soldiers have been killed in a fierce clash near the border with Afghanistan.
Eighteen other suspects were arrested in a prolonged gun battle in the district of South Waziristan.
Most of those killed or captured were foreigners, a military spokesman said.
Fierce fighting continued throughout Thursday
The rugged tribal area borders on the Afghan province of Paktika, which has become a centre for ousted Taleban fighters.
Fighting continued throughout Thursday until nightfall, with helicopter gunships backing up the troops.
BBC correspondent Zaffar Abbas, who was at the scene, said it was perhaps the fiercest armed clash between Pakistani troops and suspected al-Qaeda and Taleban militants in more than a year.
The security operation began hours before a senior American official, Deputy Secretary of State Richard Armitage, was expected in Pakistan.
But his trip has now been rescheduled for unspecified reasons.
Al-Qaeda chief Osama Bin Laden and his deputy, Ayman al-Zawahri, are often reported to be hiding somewhere along the long and remote border.
Hundreds of Pakistani troops are believed to be involved in what is being described as one of the biggest ground operations against al-Qaeda remnants in more than a year.
The Pakistani army began the operation "upon the receipt of credible intelligence about the presence of al-Qaeda elements", an army statement released on Thursday said.
The army says troops have surrounded the entire area, making it extremely difficult for the al-Qaeda fighters to escape.
Troops were said to be meeting stiff resistance.
A reporter at the scene for the Associated Press news agency said he saw the bodies of four suspected militants and that the army said eight more bodies were close by.
Women and children sheltering in compounds with the militants surrendered before the shooting started, according to the army.
The head of the operation, Major General Faisal Alavi, said: "Al-Qaeda people have taken refuge in these five big compounds. We do not know how many people are hiding there."
The operation is "against foreign elements who were most likely involved in attacks against coalition forces in Afghanistan",
military spokesman Major General Shaukat Sultan told AFP news agency.
"These foreign elements were causing nuisance to the local
The tribal region of South Waziristan is a highly conservative area and the rugged tribesmen have traditionally supported the Taleban across the border.
One Afghan district, Barmal, which has seen resurgent Taleban activity, is only 15 kilometres (nine miles) from the border with South Waziristan.
The Pakistan Foreign Ministry announced on Thursday that an expected visit by US Deputy Secretary of State Richard Armitage had been delayed.
Foreign Ministry spokesman Masood Khan said this was due to "scheduling issues" but did not elaborate.
Earlier, the US State Department said that Mr Armitage would encourage Pakistani President Pervez Musharraf to do more to stop any Taleban activity.
Afghanistan has complained that Pakistan is not doing enough to track down extremist elements.
Pakistani security forces have co-operated closely with the US in efforts to track down Osama Bin Laden and other al-Qaeda supporters in areas on the Afghan border where they could be hiding.