Britain has rejected the idea that it could release an Iranian tanker in exchange for a British-flagged vessel seized by Iran in the Persian Gulf.
"There is no quid pro quo," Foreign Secretary Dominic Raab told the BBC on July 29. "This is not about some kind of barter. This is about the international law and the rules of the international legal system being upheld and that is what we will insist on."
Tensions have soared since the United States withdrew from the 2015 nuclear deal between Tehran and world powers last year and reimposed sanctions.
In recent weeks, Iran and Britain have seized a tanker each, putting further pressure on the agreement.
Speaking to Sky News, Raab said that if Iran wants to "come of the dark and be accepted as a responsible member of the international community" it must follow international rules.
"You cannot go about detaining unlawfully foreign vessels," he said.
Raab's comments come after a second Royal Navy warship arrived in the Gulf to protect British ships.
The HMS Duncan destroyer joined the frigate HMS Montrose on July 28 to escort vessels sailing under the British flag through the Strait of Hormuz, a key shipping line that connects the Gulf and the Arabian Sea.
"Merchant ships must be free to travel lawfully and trade safely, anywhere in the world," Defense Secretary Ben Wallace said. "While we continue to push for a diplomatic resolution that will make this possible again without military accompaniment, the Royal Navy will continue to provide a safeguard for U.K. vessels until this is the reality.”
Meanwhile, Iranian government spokesman Ali Rabiei said that Britain’s proposal for a European-led maritime mission to escort tankers in the area is “provocative” and “carries a hostile message.”
"The presence of foreign forces will not help the region's security and will be the main source of tensions," according to President Hassan Rohani.
On July 4, British authorities seized the Iranian tanker Grace 1 off the overseas territory of Gibraltar over allegations that it was in violation of European Union sanctions on Syria.
In response, Iran's Islamic Revolutionary Guards Corps (IRGC) on July 19 seized the British-flagged oil tanker Stena Impero in the key shipping route.
Rohani has indicated the ship could be released if Britain returns the Grace 1.
Tehran said the Stena Impero was "violating international maritime rules," while then-British Foreign Secretary Jeremy Hunt called the seizure of the vessel "state piracy."
The tanker's Swedish owners, Stena Bulk, have said the 23 crew members, who included Russians, Latvians, Indians, and Filipinos, were in good health.