North Korea is apparently pushing ahead with preparations to launch its longest-range missile despite international appeals for restraint, a South Korean news report said Wednesday.

"Vehicles carrying equipment for a missile launch keep moving from a military plant near Pyongyang to Musudan-ri," a Seoul government source was quoted by Yonhap news agency as saying.

"It is suspected that the North is pushing forward with preparations in stages for a launch," the source said.

The defence ministry said it could not confirm the report and the National Intelligence Service had no comment.

Musudan-ri on the east coast is the site of the North's launch of a Taepodong-2 missile in 2006. It failed after 40 seconds.

US and South Korean officials said last week the communist state seems to be preparing to test the missile again. At maximum range it could theoretically target Alaska.

"North Korea keeps transporting equipment needed for a launch. It seems likely that the North will move on to assembling a missile, transporting it to a launch pad, fuelling it and firing it off," the source told Yonhap.

"Given the current pace of work, fuelling of the missile is likely to come within a month."

South Korea has said any such launch would breach United Nations resolutions passed after the last test, and the United States has said it would be provocative.

US Secretary of State Hillary Clinton, who will make her first tour of Asia next week, Tuesday warned the North against provocations.

She said the United States is hopeful the North's recent moves are "not a precursor of any actions that would up the ante, or threaten the stability and peace and security of the neighbours in the region."

Clinton was responding to a reporter who asked if there was a growing risk of military clashes since North Korea announced late last month it was scrapping peace accords with the South.