Japan, a close US ally, said on Friday it was determined not to break up a joint bid with Brazil, Germany and India - dubbed the Group of Four or G4 - seeking permanent seats on the powerful UN council.
"We cannot say 'that's right' and jump on to the US proposal as we have been in the G4 campaign," said Japanese Foreign Minister Nobutaka Machimura, adding that he told US Secretary of State Condoleezza Rice by telephone that Japan appreciated the US consideration.
"I think they threw a difficult curve ball that at first glance looks favourable yet also problematic," Machimura told a meeting of lawmakers.
Chief Cabinet Secretary Hiroyuki Hosoda also said the G4 would stick together.
"We will continue to regard the G4 proposal and cooperation as most important," said Hosoda, the Japanese government spokesman.
The United States said on Thursday that the G4 proposal, which would enlarge the Security Council from 15 to 25 members, was unwieldy. It opposed giving veto power to newcomers.
Undersecretary of State Nicholas Burns said Washington "will likely support adding two or so permanent members" and "two or three" additional non-permanent seats.
The US says it does not want new
members to have veto rights
He named only Japan as a country with US backing for a permanent seat, saying Washington would next week propose specific criteria for candidate countries.
The US also advocates extending the number of non-permanent Security Council members from the current 10 to a maximum of 13.
Its proposals would take the Security Council from its current total of 15 members up to a maximum 20.
Other nations have proposed a total of 25 members with the extension.
Japan has made winning a seat on the UN Security Council - whose set-up, giving veto power to Britain, China, France, Russia and the United States reflected the power balance in 1945 - a key goal of its foreign policy.
But China has vowed to block Japan, saying it must do more to atone for its atrocities during its invasion of Asian countries in the first half of the 20th century.
Burns also outlined on Thursday six areas for reform that the US sees as a priority.
They include reform of UN practices relating to budget, management and administration; the creation of a peace building commission for nations emerging from conflict; and for a human rights council to replace the existing Human Rights Commission.
Japan is the only country to have
US backing for a permanent seat
Washington also wants to see the creation of a democracy fund that would give priority to support for poor countries that are heading towards democracy; the adoption of a comprehensive convention on terrorism; and a number of proposals in the arena of development finance.
Anne Patterson, the US representative at the UN, is to speak on Tuesday at the UN General Assembly's general debate on the reform.
She is expected to specify the US position on all reforms - especially that of Security Council enlargement.
Patterson will also outline US criteria on the eligibility of candidate countries.