Britain ruled out a swap on Thursday of the British-flagged tanker seized by Iran and Iranian tanker captured near Gibraltar by British forces.

"We are not going to barter a ship that was detained legally with a ship that was detained illegally," said British Foreign Secretary Dominic Raab on a trip to Bangkok, adding that the rule of law and rule of international law must be upheld if nations have illegally detained a UK-flagged ship.

Iranian ship Grace 1 was accused by the government of the British territory of violating sanctions on Syria implemented by the European Union, when it was detained near Gibraltar on July 4, fully loaded with crude oil.

The seizure of the Iranian supertanker, however, was viewed by Iranian government as a breach of the 2015 nuclear deal, which European nations still party to.

When U.S. President Donald Trump announced the withdrawal from the deal over Iran's nuclear program and reimposed sanctions last year, the tension between the U.S and Iran has escalated ever since.

President Hassan Rouhani said on Thursday that Iran was ready for the worst in an uphill struggle to salvage its nuclear deal with the rest of the countries in the agreement, though the oil-producing country has retaliated by resuming uranium enrichment, which the West see as a sign of developing nuclear weapons.

According to Iran's Foreign Secretary Mohammad Javad Zarif, British-flagged tanker Stena Bulk was "violating international maritime laws" when it was seized by Iranian armed forces on July 19 near the Strait of Hormuz.

Along with several attacks on oil tankers in the past two months, the U.S. has called on European nations to form a protection mission to ensure safe shipping around the Gulf waters, whereas Britain, Germany and France are actively seeking diplomatic moves to resolve the crisis.

(With input from agencies)