TOKYO, JAN. 13 -- Japan's foreign minister flew to Washington today, reportedly to give President Bush personal assurance that Tokyo will back him in the Persian Gulf come peace or war.

Taro Nakayama was scheduled to meet Monday with the president and with Secretary of State James A. Baker III, mainly to discuss the Middle East. The Uruguay Round of world trade talks also is on the agenda, according to officials from both countries.

In addition, Baker and Nakayama are expected to sign an agreement on U.S. forces here. As previously reported, Japan agrees to raise its payment for the U.S. troops to 50 percent of costs by 1996. Currently, Japan pays about 40 percent.

But an official at the Foreign Ministry said, "The meetings will obviously deal mainly with the gulf crisis." Nakayama is expected to say Japan still hopes war can be avoided, but will back the U.S.-led allied force if hostilities occur.

Foreign Ministry officials here said they are planning to ask the Diet, or parliament, for additional funds for the allied military effort and for refugee programs.

The State Department distributed to Japanese media a new statement on U.S.-Japan relations. As reported in the press here, Baker said "America needs Japan's leadership and active support" both in the gulf crisis and the Uruguay Round.

Opinion polls indicate Japanese are not sure about the wisdom of fighting, but the government seems to have decided it will remain firmly tied to the U.S.-led effort, whether or not war breaks out.