The Japanese government has activated an emergency alert system against potential North Korean provocations with the start of South Korea-U.S. joint military drills on Sunday.

Tokyo officials at the crisis management center set up at the prime minister's office are focusing on gathering intelligence. Self-Defense Forces and electronic spy aircraft have bolstered patrol on and above sea.

Japanese Prime Minister Naota Kan has ordered all Cabinet officials to be on standby within capital Tokyo during the South Korea-U.S. exercise period from Sunday through Wednesday.

In a contingency, the officials must report to work at their respective ministries within an hour of an order directed by the chief cabinet secretary.

Japanese Foreign Minister Seiji Maehara also held a phone conversation with his Chinese counterpart Yang Jeichi on Saturday and urged Beijing's role to restrain North Korea's provocative acts.