Japan's ruling party has endorsed a draft bill to extend a refuelling mission in support of US-forces in Afghanistan and at the same time scale back Japanese support for combat operations.
But the bill, set to go to the cabinet on Wednesday, was unlikely to placate the opposition which has lobbied to end the mission entirely.
The proposed bill limits Japanese ships to refueling and supplying water to ships on patrol.
They will not refuel vessels involved in military operations, such as attacks, rescue operations or humanitarian relief.
The opposition, which controls the upper house of parliament, has not officially rejected the compromise bill, saying it wants to debate the issue once the cabinet approves it.
The upper house, dominated by the Democratic Party of Japan (DPJ) since July, has the ability to slow the ruling blocs legislative plans but not derail them.
The ruling camp could override the upper house through its two-thirds majority in the more powerful lower chamber, tough legislators may be reluctant to risk a public backlash by doing so, according to analysts.
The new bill was due to go to the Diet, or parliament, for a plenary vote, ruling party and government officials said.
"Hopefully the bill wins the Democrats' support and get passed by parliament," Taku Yamasaki, a Liberal Democratic Party executive, said.
But the opposition says that Japan should not be involved in "American wars", though it has yet to agree on a counter-proposal to the government bill.
The opposition remains divided on a compromise, with Ichiro Ozawa, who heads the DPJ, suggesting that Japan could provide support for the International Security Assistance Force in Afghanistan, a UN-authorised, Nato-led peacekeeping mission.
The current mission, started in 2001, expires on November 1.