North Korean leader Kim Jong Un (C) supervises an expanded meeting of the Political Bureau of the Central Committee of the Workers' Party of Korea in Pyongyang in this February 18, 2015 photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang February 19, 2015. REUTERS/KCNA (NORTH KOREA - Tags: POLITICS) ATTENTION EDITORS - THIS PICTURE WAS PROVIDED BY A THIRD PARTY. REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE AUTHENTICITY, CONTENT, LOCATION OR DATE OF THIS IMAGE. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS PICTURE IS DISTRIBUTED EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS. NO THIRD PARTY SALES. NOT FOR USE BY REUTERS THIRD PARTY DISTRIBUTORS. SOUTH KOREA OUT. NO COMMERCIAL OR EDITORIAL SALES IN SOUTH KOREA

By Ju-min Park

SEOUL (Reuters) - North Korea fired two short-range missiles off its eastern coast on Monday, South Korean officials said, a defiant response to annual joint military exercises between South Korea and the United States, but one which drew a swift protest from Japan.

The firing came hours before the U.S.-South Korean military exercises were scheduled to begin, drills which the secretive North denounces as a preparation for war.

The missiles landed in the sea between the Korean Peninsula and southern Japan early on Monday morning after traveling for about 490 km (305 miles), according to South Korea's Defense Ministry.

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.

A U.S. State Department spokesperson said such missile launches represent a threat to peace in the region.

The spokesperson also said the U.S. calls on North Korea to "refrain from provocative actions that raise tensions in the region."

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."

Japan quickly lodged a protest with North Korea over the latest missile launches, saying they posed a serious threat to safety at sea and in the sky.

"The ballistic missile launches by North Korea are extremely problematic conduct in terms of aviation and navigation safety," Chief Cabinet Secretary Yoshihide Suga told a news conference.

"We swiftly lodged a stern protest with North Korea."

Japan treads a fine line between conveying its condemnation to Pyongyang over such actions while not trying not to derail bilateral talks aimed at resolving the issue of Japanese citizens abducted by North Korean agents decades ago.

In July, Japan eased some sanctions on North Korea in return for the North reopening its investigation into the fate of Japanese abductees, but little progress has been made so far.

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.

(Additional reporting by Kaori Kaneko and Kiyoshi Takenaka in TOKYO and David Brunnstrom in Washington; Editing by Paul Tait and G Crosse)