Iranian state television says authorities plan to prosecute eight British sailors seized aboard three Royal Navy patrol boats on a waterway between Iraq and Iran.
British spokesmen say Foreign Secretary Jack Straw has spoken by telephone with his Iranian counterpart, Kamal Kharazi, who is promising to look into the matter of the captured sailors.
A British military spokesman in the southern Iraqi city of Basra, Major Ian Clooney, is playing down the incident.
"They were doing a routine patrol, as has happened for many months now, as part of the support and training offered to both the Iraqi civil defense corps and the riverine patrol department, so there was nothing unusual in what they were doing," he said.
Iranian Revolutionary Guards seized the sailors and their boats on Monday, claiming the British vessels had entered Iranian territorial waters on the Shatt al-Arab seaway that separates Iran and Iraq.
Iranian television later broadcast videotape of the men, and weapons and military gear allegedly captured from them.