"If there has been a coup, we would obviously seek the quickest possible restoration of democracy in Pakistan," said US State Department spokesman James Rubin.

Only three weeks ago, US officials expressed concern about a possible military coup because of political turmoil and opposition demands that Mr Sharif should leave office.

"We hope there will be no return to the days of interrupted democracy in Pakistan," a US official said then. "We would strongly oppose any attempt to change the government through extra-constitutional means."

US State Department spokesman James Rubin: "The spirit and the letter of the constitution should be respected"

The statement brought into the open coup speculation that had been rife since Mr Sharif promised President Bill Clinton on 4 July to withdraw Pakistani-backed militants from Kargil.

Critics described Mr Sharif's turnabout as a humiliation.

President Clinton has been hoping to make a trip to India and Pakistan early next year.

India army alert

India's armed forces have meanwhile been put on alert, Press Trust of India reported.

"All the three wings of the armed forces are on full alert and we are monitoring the situation closely," the news agency said, quoting a senior defence official.

India has expressed "grave concern" about reports from Pakistan of soldiers surrounding the house of Prime Minister Nawaz Sharif and key public institutions.

"Reports emanating from Pakistan are a matter of grave concern. We are watching the situation. The cabinet committee on security will meet soon after the swearing-in of the new government (on Wednesday) to consider the developments in Pakistan," a spokesman at the Prime Minister's Office said.

The statement said that Prime Minister Atal Behari Vajpayee, whose Bharatiya Janata Party was re-elected last week, was keeping itself "fully abreast of the evolving situation in Pakistan".