The state IRNA news agency quoted Foreign Ministry spokesman Muhammad Ali Hosseini as saying Britain should "accept responsibility" for an act that he said was "contrary to international treaties."
Iran seized 15 British service personnel on March 22 in the Persian Gulf waterway that separates Iran and Iraq. Britain says the 15 were in Iraqi territorial waters when they were captured.
But Iranian military spokesman Alireza Afshar today said the group had "confessed" they had been operating in Iranian waters.
The British Foreign Ministry has summoned Iran's ambassador to London for a meeting -- the second over the past two days -- over the arrests.
Iran's state television said the service personnel -- including sailors and marines -- would remain in detention for as long as investigations were going on. Iranian media said the 15 have been transferred to Tehran for questioning. Reports said at least one woman was among the group.
Meanwhile, Germany, which holds the rotating EU Presidency, has urged Iran to release the British service personnel.
German Foreign Minister Frank-Walter Steinmeier said the European Union will demand that the service personnel be immediately released.
"We have no definite hints, but have obtained official confirmation from Iran that the troops were under arrest for an alleged border violation," Steinmeier said today in Berlin. "The German EU presidency will issue a statement later today demanding clearly that Iran immediately release the soldiers."
The diplomatic crisis comes as the UN Security Council prepares to vote on whether to expand sanctions against Iran for its nuclear program.