Tokyo (AFP) - Japan is investigating nearly a dozen suspicious boats recently found drifting off the country's coastline, some with decaying bodies aboard, officials said Friday, as media speculated they came from North Korea.

At least 11 cases involving wooden boats -- some badly damaged -- with 20 bodies on board have been reported during October and November, a coastguard spokesman told AFP.

Many of the boats have been towed to Japanese ports, but the bodies are yet to be identified, he said, adding that investigations were ongoing.

On Tuesday, one of the boats was pulled ashore at Fukui port after three sets of remains were found inside when coastguard personnel spotted it some 100 kilometres (62 miles) offshore in the Sea of Japan, private TV channel Tokyo Broadcasting System reported.

The body of water is known as the East Sea in North and South Korea.

TBS said the remains were badly decomposed and partially skeletonised, while Japan's public TV broadcaster NHK said Korean writing was visible on the boats as well as clothes left inside the vessels.

Experts suspect the vessels are fishing boats from North Korea as the impoverished state prone to poor harvests is trying to expand its fishing industry to bolster food security, NHK said.

But a number of North Korean fishing boats have been lost due to a lack of modern equipment, including the Global Positioning System capability, it added.