(Adds detail, background, previous ISLAMABAD)

PESHAWAR, Pakistan, Dec 11 (Reuters) - A missile strike, suspected to be from a pilotless U.S. drone, killed seven militants on Thursday in Pakistan's South Waziristan region, seen as an al Qaeda sanctuary, two intelligence officials said.

One of the officials said there may be foreigners among the dead but that he did not know their nationalities.

"The missile hit a house adjacent to a madrasa (Islamic seminary). Seven people are killed," he said.

"Most of those killed are Punjabis," the official added, employing a term used for militants from Pakistan's central province of Punjab.

It was the second suspected strike by U.S. drone aircraft this month, after three people were killed in a similar attack in neighbouring North Waziristan.

There have been over 20 strikes in the last three months in the tribal regions and nearby areas. They reflect U.S. impatience over support provided by militants from Pakistan for the Taliban insurgency in Afghanistan, and fears that al Qaeda fighters in northwest Pakistan could plan attacks in the West.

Rashid Rauf, a British militant with al Qaeda links, was killed in a similar strike in Mir Ali last month, together with an Egyptian, intelligence officials say.

The latest strikes came amid growing tension between nuclear-armed Pakistan and India over deadly attacks on Mumbai last month.

India has blamed militants based in Pakistan for the attacks on two luxury hotels, a railway station and a Jewish centre in its financial capital that killed at least 179 people.

Pakistan has condemned the attacks, promised full cooperation in investigations and launched a crackdown on an Islamist charity regarded as a front for Lashkar-e-Taiba, the militant group suspected of being behind the operation. (Writing by Zeeshan Haider, editing by Mark Trevelyan)

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