SEOUL, South Korea — The South Korean government warned on Sunday that the North might launch a missile later this week, while a top military leader postponed a scheduled trip to Washington, citing escalating tensions on the peninsula.

The warning by Kim Jang-soo, director of national security for President Park Geun-hye, came three days after the South Korea’s defense minister said that the North had moved to its east coast a missile with a “considerable range” but not capable of reaching the mainland United States.

The missile was widely believed to be the Musudan, which the South Korean military says can travel “more than” 3,000 kilometers or 1,864 miles. But South Korean media and analysts say the missile can extend its range to 4,000 kilometers or 2,490 miles, which would put American bases in Guam within its reach.

Mr. Kim said that the North Korean authorities had told foreign embassies in Pyongyang to inform them by Wednesday whether they needed assistance in evacuating should they wish to because of rising tensions on the peninsula.