Story highlights The South Korean Defense Ministry is on high alert

The launch is the second in less than a week

They come after the start of joint military exercises between U.S. and South Korea

North Korea fired two short-range missiles off its eastern coast Monday, the second such launch in less than a week, according to the South Korean Defense Ministry.

The defense ministry said it is on high alert and monitoring the situation. It called on the North to stop the missile launches.

The weapons launched were Scud missiles that flew more than 500 kilometers (311 miles), according to the defense ministry. The missiles landed in the sea, South Korea's semi-official Yonhap news agency reported.

On Thursday, four Scud missiles with a shorter range were fired into the sea off North Korea's eastern coast -- flying about 220 kilometers (137 miles), according to Yonhap -- just days after the start of annual joint military exercises between South Korea and the United States. North Korea opposes such exercises, which routinely cause friction among the three countries.

Last year's exercises triggered weeks of heightened tensions between the nations and North Korean threats of nuclear war.

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Thursday's launch was the first time North Korea had fired Scud missiles, which have a range that covers the whole of the Korean Peninsula, since 2009, South Korea said.

Foreign policy experts say the North Korean missile firings may not herald a repeat of last year's saber rattling from Pyongyang, which included threats of preemptive nuclear strikes against the United States and South Korea and the declaration that the armistice that stopped the Korean War in 1953 is null and void.