Peace protestors oppose Japanese involvement
He wants to send forces to provide medical supplies and services, and intelligence-gathering in case of any retaliatory action.
Meanwhile, China has urged Japan to be prudent, warning that the issue was still a very sensitive one.
And a commentary on North Korean state-run radio has criticised Japan of taking advantage of the situation to expand its power within the region.
Public support... but
Mr Koizumi told parliament on Thursday of his outrage, during a visit to the US this week, at the "inhuman act" of the attacks on New York and Washington.
He promised to "create as quickly as possible the necessary framework to implement... measures
I announced last week".
Two opinion polls, published on Wednesday, showed 90% support among the Japanese public for action to help the US.
But analysts warn the prime minister has to tread carefully to maintain that support.
They point to the same opinion polls showing less than 10% support for actually sending Japanese troops into combat.
Megawati meeting
Mr Koizumi is due to meet another regional leader struggling to strike the right balance between international commitments and conflicting demands at home, Indonesian President Megawati Sukarnoputri
She too has recently returned from a trip to Washington, where she condemned the attacks.
But President Megawati faces strong pressure in Indonesia - the world's most populous Muslim country - where some protestors have demonstrated against any military action against Afghanistan.
She is also meeting business leaders during her visit. Japan is one of Indonesia's main trading partners, as well as its leading aid donor.
Constitutional changes
The Japanese Government wants to make changes to the Self Defence Forces Law.
Drawn up by the US at the end of World War II, it bans the "threat or use of force as a means of settling international disputes" and bars this country from maintaining "land, sea and air forces, or other war potential" to that end.
Since 1954, that has been interpreted to allow for defensive forces and Japan now maintains a large and well-equipped military, called the Self-Defence Forces (SDF).
Thursday' debate will discuss freeing up the SDF to allow it to protect US bases in Japan - which it currently cannot do - and pave the way for legislation, being prepared for discussion next month, to allow Japanese forces to provide rearguard support such as medical services and transporting supplies.
Up to four Japanese ships, including a state-of-the-art destroyer are reported to be preparing to sail, depending on the outcome of Thursday's debate, to rendezvous with a US fleet in the Indian Ocean.
The Japanese Government appears determined to avoid the humiliation it suffered during the Gulf War, when it contributed money but no troops and received little thanks from the participants.
But any constitutional changes freeing up Japan's military is sure to raise concerns among neighbouring countries who have previously suffered under Japanese wartime military aggression.