By Sanjeev Miglani

SINGAPORE, Jan 11 (Reuters) - Pakistan President Pervez Musharraf has said any unilateral action by U.S.-led coalition forces against militants in the border region with Afghanistan will be regarded as an invasion, a newspaper reported on Friday.

Musharraf told Singapore's The Straits Times that Islamabad will resist any entry by coalition forces in the tribal areas to hunt down Islamic militants, regarding that as a breach of Pakistan's sovereignty.

"I challenge anybody coming into our mountains. They would regret the day," he told the newspaper in an interview conducted in the garrison city of Rawalpindi.

Musharraf also told the newspaper he would resign if a government that emerged from elections, now scheduled for next month, sought his impeachment.

Pakistan has been under pressure from Washington to stamp out al Qaeda and pro-Taliban militants U.S. officials believe are hiding in remote regions along the border with Afghanistan and fomenting violence there.

The New York Times said earlier this month the U.S. government was considering expanding the authority of the CIA and the military to conduct aggressive covert operations in Pakistan.

Pakistan officials dismissed the report at the time and said Islamabad would not permit any such action.

Musharraf also criticised U.S. Democratic presidential front-runner Hillary Clinton's proposal to deploy a U.S., and possibly a British, team to safeguard Pakistan's nuclear assets.

Her statement, Musharraf said, was an "intrusion into our privacy, into our sensitivity ... She doesn't seem to understand how well-guarded these assets are."

International concern about the safety of Pakistan's nuclear weapons has been increasing, and earlier this week, Mohamed Elbaradei, the head of the International Atomic Energy Agency, was quoted as saying he feared the nuclear arsenal could pass into the hands of extremists.

Musharraf is also under fire at home.

The Pakistan People's Party led by Benazir Bhutto until her assassination last month has been attacking him over her killing while Nawaz Sharif, another former premier, has demanded his immediate resignation and the formation of an all-party government to conduct free and fair elections.

"If impeachment were their intention and they don't want to go along in a harmonious manner, I would like to quit the scene," Musharraf said when asked what would happen if Bhutto's party emerged a winner in the elections and mounted a bid to oust him with the support of Sharif's party.

"If that happens, let me assure you that I would be leaving office before they would do anything." (Editing by Jerry Norton)

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