US President George W Bush described the 15 British navy personnel held by Iran as "hostages" and said that their capture was "inexcusable".
His comments on Saturday at the presidential retreat in Camp David, Maryland, were his first on the issue since the 15 Britons were seized from a boat in what London insists was Iraqi waters.
Tehran alleges that the British Navy vessel was in Iranian waters on March 23.
"The British hostages issue is a serious issue because the Iranians took these people out of Iraqi water, and it's inexcusable behaviour," Bush said.
"I strongly support the Blair government's attempts to resolve this peacefully, and I support the prime minister (Tony Blair of Britain) when he made it clear there were no quid pro quos. The Iranians must give back the hostages. They're innocent. They were doing nothing wrong."
Previously, a White House spokesperson and US Secretary of State Condoleezza have voiced support for Britain and called on Iran to release the British detainees.
Meanwhile, Iranian President Mahmoud Ahmadinejad once again said that Britain apologise to Iran for having violated Iranian waters.
"The British occupying forces have entered into our waters and our border guards have arrested them courageously. However, the arrogant powers, because of their arrogance and egoistic mentality, do not apologize to the Iranians, (and) rather act as if we owe them something," Ahmadinejad said in Andimeshk town in the southern Iranian province of Khuzestan.