A White House spokesman, Gordon Johndroe, said the United States was in touch with both countries and urging greater cooperation in investigating last month's Mumbai attacks, which India blames on Pakistan-based militants.

Witnesses say they saw Pakistani troops moving eastward from Pakistan's border with Afghanistan, where soldiers have been fighting al-Qaida and Taliban militants.

The military also canceled leave for members of the armed forces, and put both the army and air force on high alert.

Despite the moves, Pakistan's Prime Minister Yousaf Raza Gilani said again Friday that his country would take no aggressive action and only respond if provoked.

Indian Foreign Minister Pranab Mukherjee told reporters in New Delhi that Pakistan was trying to "divert attention" away from targeting militant groups.

India's Foreign Ministry on Friday advised Indian citizens that it is "unsafe for them to travel or be in Pakistan."

The ministry linked its warning to the reported but unconfirmed arrests of three Indian citizens in connection to a bombing in Lahore on Wednesday -- news that has been greeted with skepticism by the Indian press.

Indian Prime Minister Manmohan Singh discussed the growing tensions in a meeting with his military chiefs on Friday.

New Delhi says the militants who laid siege to Mumbai were Pakistanis, trained and equipped by the Pakistan-based militant group Lashkar-e-Taiba. Islamabad has demanded proof of the gunmen's nationality. The November 26 attacks killed more than 170 people in India's financial capital.

Nuclear-armed India and Pakistan have fought three wars since their independence from Britain in 1947.