AFP - North Korea appears to be preparing to test-fire a long-range missile, South Korea's defence ministry said Monday, amid reports that a launch could come within weeks.

"We have detected signs that North Korea is preparing to fire an ICBM (intercontinental ballistic missile)," a defence ministry spokesman told AFP.

"US and South Korean intelligence authorities are still analysing the information," he said, declining to confirm a report by South Korea's Yonhap news agency that the launch could take place in one or two weeks.

The North has moved a long-range missile to a launch base in Dongchang-ri along its northwestern coast, Yonhap quoted intelligence sources as saying.

The North has another site at Musudan-ri on the east coast, from where it fired a long-range rocket over Japan on April 5. It said it put a satellite into orbit but other nations saw the launch as a disguised missile test.

Tensions have been running high for the past week after North Korea tested a nuclear bomb for the second time and launched a series of short-range missiles.

Pyongyang has warned it would take "additional self-defence measures" in response to any tougher international sanctions over its nuclear test.

South Korean and US forces are on heightened alert for border clashes after the North warned of a possible attack in response to Seoul's decision to join a US-led initiative to stop the spread of weapons of mass destruction.

North Korea has stepped up military activities, particularly near the western sea border, the site of bloody naval clashes in 1999 and 2002, another defence ministry spokesman said.

"North Korean troops have been conducting more drills, especially along the west coast," he told AFP.

North Korea has conducted shooting drills or used high-speed boats in the area for landing exercises and has banned ships from some areas of the Yellow Sea until the end of July, Yonhap reported.