The British government says Iran's delay in providing a full explanation for why it detained five British yachtsmen last week is a "matter of increasing concern."
Britain's Foreign Office says it has been trying to confirm where the sailors are being held.
The head of Britain's diplomatic service, Peter Ricketts, met Tuesday with Iran's ambassador to London, Rasoul Movahedian, and requested that Iran allow consular officials to visit the five sailors.
Iranian naval forces detained the Britons last week as they sailed from Bahrain to Dubai for a race. British officials say the yacht "may have strayed inadvertently" into Iranian waters.
The chief of staff to Iran's President Mahmoud Ahmadinejad, Esfandiar Rahim Mashaie, told Fars news agency Tuesday that Iran's judiciary will decide what to do with the five Britons. He said strong measures will be taken if it is proven that they had "bad intentions."
British Foreign Secretary David Miliband denied the sailors had any malicious intent. He said the incident has "nothing to do" with politics or Iran's nuclear program.
Iranian Nobel Peace Prize laureate Shirin Ebadi has urged the Iranian government to free the five sailors.
Speaking to reporters in Seoul, South Korea, the human rights lawyer said Iranian authorities should have first warned the yacht to leave the area.
In March 2007, Iran detained 15 British military personnel in disputed waters and held them for about two weeks, sparking a diplomatic stand-off.
Tehran maintained the Britons had entered Iranian waters, while Britain insisted they were in Iraqi waters.
Iran is holding three U.S. citizens who were detained July 31 for entering Iran illegally, after they apparently strayed across the border while hiking in northern Iraq. Iran has charged the three Americans with espionage.
Some information for this report was provided by AFP and Reuters.