NKorea bans ships off port from Wednesday

North Korea is warning ships to stay out of waters off its eastern port of Wonsan for three weeks from Wednesday, the Japan Coast Guard said, raising concerns Pyongyang is planning more missile tests.

North Korea is alerting vessels by radio not to enter an area that measures 100 by 263 kilometres (60 by 165 miles) at its widest points from June 10 to 30 between 8:00 am and 8:00 pm, a coast guard spokesman said Monday.

"We have acknowledged the information and soon afterward issued the same warning to those who may travel in this region," the spokesman said.

The news came amid increasing speculation that North Korea is preparing to test-fire several medium-range missiles from its southeast coast.

At least three missiles are apparently being prepared for launch from a missile base in Anbyon County, near Wonsan, a port city about 100 kilometres northeast of Seoul, South Korea's Yonhap news agency reported Tuesday.

The report said that several vehicles mounted with mobile launch pads were spotted at the base.

The Japan Coast Guard picked up a similar warning in May, only days before Pyongyang conducted its second nuclear test and also launched a series of short-range missiles.

"We can't deny the possibility that North Korea is moving toward launching missiles, including ballistic missiles, in response to developments related to a UN Security Council resolution," said Chief Cabinet Secretary Takeo Kawamura.

"We are doing everything to secure the peace and security of Japan and its people, while collecting and analysing information."

South Korean and US forces on the peninsula are on heightened alert after the North threatened a possible attack in response to Seoul's decision to join a US-led initiative to halt the trade in weapons of mass destruction.

The North has also warned of "self-defence measures" in response to any tougher international sanctions.