US Secretary of State Colin Powell says Japan must consider revising its pacifist constitution if it wants a permanent UN Security Council seat.
Japan's sophisticated military is currently very restricted
Article Nine of the constitution, drawn up under US post-war occupation, renounces the use of force in disputes.
Japan plays a role in international peacekeeping, and currently has troops in Iraq, but its constitution limits its military's powers.
However, revising Article Nine would be highly controversial in Japan.
Mr Powell told Japan's Kyodo news agency that the US supported Tokyo's quest for a permanent seat at the Security Council.
PACIFISM UNDER THREAT?
Japan's constitution renounces the use of force
This has been stretched to allow self-defence troops
1992 law allowed troops to join UN and relief work overseas
2003 law said troops could go to non-combat zones in Iraq
PM Koizumi wants to give Japan even greater powers
But he added that: "If Japan is going to play a full role on the world stage and
become a full active participating member of the Security Council, and have the kind of obligations that it would pick up as a member of the Security Council, Article Nine would have to be examined in that light."
He acknowledged how important Article Nine was to the Japanese people, saying:
"Whether or not Article Nine should be modified or changed is absolutely and entirely up to the Japanese people to decide."
Japan says it is eligible for permanent membership as it has been involved in several peacekeeping operations, and is the second-largest contributor to the United Nations.
Mr Powell's comments reiterated remarks made by Deputy Secretary of State Richard Armitage in July.
Many Japanese are in favour of pacifism
Mr Armitage told a group of Japanese lawmakers that it would be difficult for Japan to become a permanent member of the Security Council if it could not have a greater military role in international peacekeeping.
Article Nine of Japan's post-war constitution technically forbids Japan even having a military, although this has been re-interpreted to permit forces for self-defence. Japan's government needed to pass special legislation to allow the Iraq despatch.
Both the ruling Liberal Democratic Party and the opposition Democratic Party are in favour of a change in the constitution, but many lawmakers and members of the public are unwilling to renounce Japan's pacifist stance.
A poll published in May in the Mainichi Shimbun newspaper showed that 78% of Japanese lawmakers were in favour of making some changes to the constitution, but 70% were against alterations to Article Nine.
Iran oil deal
In his comments on Thursday, Mr Powell also urged Japan to reconsider its oil development deal with Iran.
"It seems clear to us that Iran is trying to develop a nuclear weapon," Mr Powell said.
He added that he hoped Japan would take this into account when judging whether Iran was a place to invest in or do business with.
Mr Powell also reiterated that the US intended to take legal action against Charles Robert Jenkins, an alleged US Army deserter now in Japan for medical treatment.
"We cannot set aside the fact that because he is a deserter we need to resolve his case at some point in the future," Mr Powell said, adding that the US would not press the case while Mr Jenkins was under medical care.