BANGKOK — Britain on Thursday ruled out exchanging an Iranian tanker detained by Gibraltar for a British-flagged tanker seized by Iran in the Gulf.

"We are not going to barter: if people or nations have detained UK-flagged illegally then the rule of law and rule of international law must be upheld," Foreign Secretary Dominic Raab said while on a trip to Bangkok.

"We are not going to barter a ship that was detained legally with a ship that was detained illegally: that's not the way that Iran will come in from the cold," he said. "So I am afraid some kind of barter or haggle or linkage is not on the table."

Tensions have spiked between Iran and Britain since after Iranian commandos seized a British-flagged tanker last month. That came after British forces captured an Iranian oil tanker near Gibraltar, accused of violating sanctions on Syria.

Meanwhile in another development, Royal Dutch Shell announced on Thursday that is not taking any British-flagged tankers through the Strait of Hormuz.

"There are Shell-managed vessels in the Strait of Hormuz and that will probably continue to be the case. Currently, though, there are no UK-flagged vessels," Chief Executive Ben van Beurden said.

If the Anglo-Dutch oil and gas company does use a British-flagged vessel in the future, it will be accompanied by the Royal Navy as a precaution, van Beurden added.

Shell's rival BP on Tuesday said it had not taken any of its own tankers through the Strait of Hormuz, along which about a fifth of the world's oil passes, since a July 10 attempt by Iran to seize one of its vessels. — Agencies