Japanese and South Korean news reports say North Korea appears to be preparing two sites from where a long-range missile could be launched.
South Korea's Chosun Ilbo newspaper quotes an unidentified government official as saying that a special train used to carry a long-range missile has been spotted on the move.
The report says the train has been seen moving from a missile launch sites on North Korea's northwestern coast to its northeastern coast. A similar report was carried by Japan's Sankei newspaper.
South Korean intelligence and defense officials said they could not confirm the reports.
North Korea has previously used the site on its east coast for three other long-range missile launches, one in 1998, another in 2006 and the latest on April 5 of this year.
That launch sparked international condemnation. And in response the North has been lashing out against international criticism, making threats and conducting its second underground nuclear test on May 25.
U.S President Barack Obama met with his South Korean counterpart Tuesday to discuss the recent rise in tensions.
After meeting with South Korean President Lee Myung-bak Tuesday, Mr. Obama warned that a nuclear-armed North Korea is a "grave threat" to the world.
He also said he will end what he calls North Korea's cycle of starting a world crisis and then being rewarded with food and fuel for backing down, only to start up again.
The president promised there will be vigorous enforcement of United Nations sanctions against the north, which were approved late last week in response to Pyongyang's nuclear test.
Mr. Lee said South Korea's strong alliance with the United States will make North Korea think twice before making any belligerent moves.