Some believe their relatives could still be alive in the North

About 100 relatives and supporters are demanding that Japan impose economic sanctions against Pyongyang.

North Korea has admitted kidnapping 13 Japanese to help train its spies, allowing five to return home, while saying the rest had died.

But Japan believes some could still be alive and living in North Korea.

The protesters held placards and wore the blue ribbon-shaped pins that have marked their campaign to pressure Pyongyang into accounting for the missing family members that were abducted in the 1970s and 80s.

JAPAN'S MISSING Snatched in the '70s and '80s Used as cultural trainers for N Korean spies Five allowed home in 2002 Five children now freed from N Korea Eight said to be dead, others missing Heartbreak over Japan's missing

Among them are Shigeru and Sakie Yokota, whose daughter was kidnapped by North Korean agents in 1977, at the age of 13.

According to North Korea, she committed suicide in 1993.

But Japanese officials have said that tests done last year on human remains handed over by Pyongyang showed they were not Megumi's.

Her brother, Takuya, urged Mr Koizumi to punish the North with sanctions.

"We've waited for all these years, as the prime minister favours dialogue rather than pressure. But we can't even have a dialogue [with the North]," the Associated Press quoted him as saying.

"We have a legitimate weapon, a sanction, and many Japanese public support it. Mr Koizumi, please listen to our desperate plea."

Japan has already suspended food aid, but still allows limited trade with North Korea.

The government says cutting all ties would only provoke the communist state at a time when the international community is trying to coax it back to talks over its nuclear programme, according to the BBC's Tokyo correspondent Jonathan Head.