TOKYO - TOKYO may divert a naval vessel from an Indian Ocean mission to guard Japanese ships seen as under threat from pirates off Somalia, a newspaper reported on Wednesday, as China readies to send destroyers to the area.

Japan's top government spokesman said no such decision had been made, but added that Tokyo needed a plan soon on how it could contribute to fighting piracy in one of the world's busiest shipping lanes.

The issue is a headache for Japan, whose post-World War Two pacifist constitution strictly limits its military activities overseas.

The Sankei newspaper reported Japan would order a vessel taking part in a refuelling mission in support of US-led military operations in Afghanistan to escort Japanese ships, if the government decided that there was a serious threat of attack.

The government would also allow an armed counter-attack if Japanese ships were assaulted by pirates, the Sankei said.

Limiting operations to the protection of Japanese vessels would avoid a breach of current legislation. Diverting a ship already in the Indian Ocean would save time, since a dispatch from Japan would take about a month, the paper said.

In a separate report, Kyodo news agency said Japan was preparing to provide patrol boats and other vessels to Yemen for use in anti-piracy operations off Somalia.

Takeo Kawamura, the top government spokesman, said the government was still considering its options.

'There are many Japanese ships there and companies with ships are worried,' Kawamura told a news conference. 'Groups have approached us, so we need to hurry (with a response).'

Nato ships began anti-piracy operations off the Somali coast in late October, but they have failed to stop hijackings. The UN Security Council has authorised countries that have the permission of Somalia's transitional government, to enter Somali waters to pursue and attack pirates.

Two Chinese destroyers and a supply vessel are due to set out this week to join the multi-national anti-piracy operation, in a move symbolic of Beijing's growing clout. India has already deployed a warship and a South Korean official has said Seoul was likely to add its own warships to the area.

Media have reported that Japan is considering a bill that may broaden the navy's remit to allow it to play a bigger role in the international operation.

But such legislation is likely to face resistance from opposition parties, which control the upper house of parliament and can stall bills. -- REUTERS