SEOUL, South Korea — North Korea declared the 1953 Korean War armistice nullified on Monday, following through on a longstanding threat that it renewed last week amid rising tensions with South Korea.

The move comes as the United States and South Korea are in the midst of two months of joint military drills, which started on March 1, and on Monday they began another planned joint exercise that involved bringing 2,500 troops from the United States. Stirring up a sense of crisis among its impoverished people, North Korea was also staging an unusually vigorous military drill.

However, there were no signs of hostility along the border. South Korean officials said they were increasing their vigilance amid fears that North Korea might use the United States-South Korean drills and a fresh round of United Nations sanctions as an excuse to create an armed skirmish.

“We must deal strongly with a North Korean provocation,” the South’s new president, Park Geun-hye, said during her first cabinet meeting on Monday. She called for the protection of people living on a border island that was attacked by North Korean artillery in 2010 and of South Koreans working in a joint industrial park in the North Korean border city of Gaesong.