Japan has refueled warships from the U.S.-led coalition fighting in Afghanistan since 2001, but withdrew the mission in November when the opposition blocked its extension.
U.S. Ambassador Thomas Schieffer said Japan would have to decide whether it wants to be part of the international effort to combat terrorism or not.
"It would be a real tragedy if somehow Japan tried to opt out of the war on terror," he told reporters at his Tokyo residence. "It’s just an issue that requires a unified international community to make any headway against."
Japan’s ruling party last week extended the current session of parliament in an effort to push through legislation that would renew the mission but prevent Japan from supporting ships directly related to the U.S.-led coalition in Afghanistan.
Under that version of the bill, the Japan would be limited to refueling ships used in monitoring and inspecting vessels suspected of links to terrorism or arms smuggling.
Schieffer has been a vocal advocate of the bill, meeting with opposition lawmakers and others to argue for its passage.
The Afghan mission was part of Japan’s high-profile response in support of the United States following the Sept. 11, 2001, terrorist attacks in New York and Washington.
Japan also dispatched a humanitarian ground unit to southern Iraq in 2004-2006, and still has a logistical unit in Kuwait.
The United States bases 50,000 troops in Japan under a security treaty, and Schieffer said he had hoped the strength of the U.S.-Japan alliance would take precedence over politics.
"It’s a little disappointing to see that somehow the alliance has become the focus of a political fight between the two parties," he said.