North Korea carried out a "failed" Taepodong-2 missile test in 2006
North Korea appears to be preparing to test-fire its longest range missile, apparently capable of hitting the US, a South Korean official and reports say.
Satellite spy images reportedly showed a train loaded with a large cylindrical object, moving towards a missile launch site close to the Chinese border.
So far, there has been no on-the-record confirmation from any government.
It follows a recent declaration by the North that it was scrapping pacts with the South made to prevent hostilities.
Reports of the apparent missile preparations were carried in both the South Korean and Japanese press, quoting government sources.
Experts quoted in the reports said any such test would not take place for at least another month.
There was similar media speculation ahead of North Korea's last long-range missile test in July 2006.
The Taepodong-2 missile, thought to have a potential range of more than 6,000km (3,730 miles), failed soon after launching. Pyongyang tested a nuclear bomb in the same year.
Relations between the Koreas have deteriorated since the South's President Lee Myung-bak, who came to power nearly a year ago, took a harder line approach to the North.
He has promised to stop the free flow of aid to the North unless it moves to end its nuclear weapons programme - a move the North says has pushed relations "to the brink of war".
Dec 07: Lee Myung-bak wins South Korean presidential election. Vows tougher line on the North
March 08: North expels S Koreans from joint industrial park after Seoul says it will link its aid more closely to the nuclear disarmament issue
April 08: N Korean media warns President Lee his tough stance could have "catastrophic consequences"
July 08: Pyongyang rejects President Lee's offer of direct talks
Oct 08: Military officials from both sides hold first direct talks since President Lee took office
Nov 08: N Korea says it will close land borders, suspend tourism trips and the joint train service because of "relentless confrontation" from Seoul
Dec 08: N Korea enforces stricter border controls and expels hundreds of South Koreans from the joint industrial zone
Jan 09: The North says it is scrapping all military and political pacts signed with the South
A statement carried by Pyongyang's official news agency said it was scrapping all political and military agreements with the South, including one covering the de facto maritime border in the Yellow Sea, over which the two countries have fought bloody skirmishes.
Seoul expressed regret at the move, while the US called it "unhelpful".
Analysts believe that Pyongyang is trying to build up tensions with the South in order to give itself more negotiating power with the new US administration.
US President Barack Obama has told his South Korean counterpart that Secretary of State Hillary Clinton would visit Seoul in mid-February.
The two Korean states are still technically at war because their three-year conflict ended in a truce, not a peace treaty, in 1953.
The peninsula remains divided by a heavily fortified Demilitarised Zone, with thousands of troops stationed on both sides of the border.