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SEOUL, South Korea (CBS NEWS) -- A U.S. Marine CH-53E "Super Stallion" helicopter made what the U.S. military headquarters in Seoul is calling a "hard landing' south of the demilitarized zone separating North and South Korea Tuesday.

The military says all 21 on board, including 5 crew members, were brought to a hospital. Fifteen were released, and those still there are in stable condition, the military says.

The cause of the crash was being investigated.

The military says the chopper was taking part in joint U.S.-South Korean war games.

Those exercises have led to a torrent of belligerent rhetoric from North Korea.

The crew is stationed in Okinawa, Japan, the military adds.

In the North, Pyongyang quietly marked a second day of celebrations for its first leader's birthday Tuesday and issued prickly new rhetoric threatening retaliation for what it sees as provocations by South Korea and the U.S., who have been watching closely for signs North Korea will go ahead with a suspected plan to test a medium-range missile.

State media said the Supreme Command of the Korean People's Army issued an ultimatum demanding an apology from South Korea for "hostile acts" and threatening that unspecified retaliatory actions would happen at any time.

The statement followed a day of festivities in North Korea's capital that featured art performances, public dances and crowds thronging to giant bronze statues to pay homage to the late leader Kim Il Sung.

The angry ultimatum, relayed through the Korean Central News Agency, was sparked by a protest of about 250 people in downtown Seoul, where effigies of Kim Il Sung and his son and successor, the late leader Kim Jong Il, were burned. One protester carried a placard saying "Kim Jong Un Out." Such protests are not unusual in South Korea and this one was likely more of a pretext for the North to react to calls that it join in dialogue with its neighbors than an actual cause for retaliation.

READ MORE: http://www.cbsnews.com/8301-202_162-57579764/u.s-marine-chopper-in-hard-landing-near-n-korea-border/

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