The tough comment came after Britain demanded the return of the sailors and denied they had strayed into Iranian waters while searching for smugglers off Iraq's coast.

The eight Royal Navy sailors and seven Royal Marines had just searched a merchant ship when they and their two inflatable boats were intercepted by Iranian vessels on Friday near the disputed Shatt al-Arab waterway, US and British officials said. The Iranian vessels surrounded them and escorted them away at gunpoint.

The incident came at a time of heightened tensions over Tehran's nuclear ambitions and allegations that Iran is arming Shi'ite militias in Iraq, but Britain was treating it as a mistake rather than a provocation.

Iranian foreign ministry spokesman Mohammad Ali Hosseini said Iran was carrying out a "further investigation of the blatant aggression. . . Violating the sovereign boundaries of other states and illegal entry denote unusual goals in violation of international commitments, the responsibility for which cannot be evaded under any justification, " Hosseini said.

He accused Britain of trying to cover up the illegal entry. "The British officials, instead of making up for their blunders, should try to refrain from putting the blame on others by way of irrelevant interpretations, " he said.

Iran summoned the British charge d'affaires to the foreign ministry on Friday, demanded an immediate explanation and "asked that this not happen again, " according to Iran's state-run television.

Britain, in turn, demanded that Tehran release the 15. In London, the government summoned the Iranian ambassador to the foreign office, and foreign secretary Margaret Beckett said he "was left in no doubt that we want them back".

Britain's defence said the sailors were in Iraqi territorial waters when they were seized. But the Iraqi military commander of the country's territorial waters cast doubt on those claims. The sailors are part of a task force that maintains security in Iraqi waters under authority of the UN security council.