Blair was speaking at a joint news conference in Washington with U.S. President George W. Bush, after the two held a farewell summit at the White House.
"I believe that we will remain staunch and steadfast ally in the fight against terrorism in Iraq, and Afghanistan, and elsewhere," Blair said, adding that he did not regret his decision to support the U.S.-led invasion of Iraq.
Blair also said that U.S.-British relations are not only shaped by the past but also built on common future interests.
Blair is due to step down on June 27, when he will be succeeded by Chancellor of the Exchequer Gordon Brown.
On other issues, Bush said that the United Nations should consider imposing new sanctions on Iran if it refuses to accept international demands to stop its nuclear program.
The UN Security Council has imposed two rounds of sanctions on Iran since December because of its continuing nuclear activities.
Bush and Blair also expressed frustration over the international community's failure to act against Sudan over the bloodshed in Darfur.
(with agency material)