Shinzo Abe, Japan's prime minister, offered his support in a phone call to George Bush from Berlin.
His foreign minister, Taro Aso, said: "Japan will continue to closely communicate and co-operate with the US."
Roh Moo-hyun, South Korea's president, also spoke with Bush by telephone before the announced policy shift and expressed his support, Roh's office said.
John Howard, the Australian prime minister, also endorsed Bush's decision.
Güncelleme Tarihi: 20 Eylül 2018, 18:16
"If America retreats in Iraq, then that has enormous consequences for the stability of the Middle East and it will also be an enormous boost to terrorism in our part of the world," he said in Sy
But Howard said he did not "see any need at the present time" to add to his country's 1,300 troops in and around Iraq.
The US president announced on Wednesday the deployment of 21,500 additional US troops to Iraq while making a rare acknowledgment of past failures in his policy.
Japan withdrew its 550 non-combat ground troops from southern Iraq last year, but has continued air support and Abe has made firm support for Bush a cornerstone of his foreign policy since taking office in September.
South Korea has 2,300 troops in the northern Iraqi city of Irbil to support the US-led reconstruction of Iraq.
Seoul's contribution of forces makes it Washington's biggest coalition partner after Britain. Seoul plans to withdraw 1,100 troops by April, and parliament has instructed the administration to devise a full pullout plan in 2007.