SEOUL (Reuters) - North Korea fired two short-range missiles off its east coast on Monday, the South Korean military said, a move seen as a protest against annual military exercises between South Korea and the United States that were due to start hours later.

The missiles hit the sea early on Monday morning after traveling for about 490 km (305 miles), according to South Korea's defense ministry.

The firing came on the day when the U.S.-South Korean military exercises were scheduled to begin. The secretive North denounces the drills as a preparation for war.

South Korean Defense Ministry spokesman Kim Min-seok said North Korea fired the missiles without designating any no-sail zones, which was regarded as a provocation.

"If North Korea takes provocative actions, our military will react firmly and strongly so North Korea will regret it in its bones," Kim told a news briefing.

Pyongyang has escalated its rhetoric against the drills, with a spokesman for its army general staff saying Washington and Seoul "should be dealt with only by merciless strikes".

North Korea frequently tests short-range missiles off its coast as part of military drills.

The United Nations has imposed sanctions banning North Korea from using ballistic missile technologies.