Sri Lanka's government believes its soldiers are on the verge of defeating the Liberation Tigers of Tamil Eelam (LTTE) after 37 years of conflict.
At the height of their power in 2006, the Tigers -- who want an independent Tamil homeland in the Sinhalese-majority island -- controlled roughly a third of the island.
The Tigers have since been driven back into a sliver of land on the northeastern coast, where the UN has accused them of holding up to 50,000 Tamil civilians hostage.
Sri Lankan leaders have refused all international calls for a ceasefire, despite reports from the UN last month saying up to 6,500 civilians may have been killed and 14,000 wounded in fighting since January.
Japan, which is Sri Lanka's largest aid donor, must "shoulder its responsibilities" and confront the worsening humanitarian crisis there, human rights and conflict prevention groups said Monday.
The appeal was made in a joint letter to Japanese Prime Minister Taro Aso from the heads of Human Right