LONDON (Reuters) - Gibraltar took the decision to seize an Iranian tanker last week solely because the ship was in breach of European Union sanctions and not on the request of any other country, the British territory’s chief minister said on Friday.

The Grace 1 was seized by British Royal Marines off the coast of Gibraltar on suspicion of violating sanctions against Syria.

“The decisions of Her Majesty’s government of Gibraltar were taken totally independently, based on breaches of existing law and not at all based on extraneous political considerations,” Fabian Picardo told Gibraltar’s parliament.

“These important decisions about breaches of our laws were not decisions taken at the political behest or instruction of any other state or third party.”

Iran has demanded that Britain release the ship and denies that it was taking oil to Syria in violation of sanctions. The affair has led to an increase in tension in the Gulf, with Britain saying on Thursday that it fended off Iranian ships that tried to block a British tanker.

Tehran blames the United States for arranging to have its ship seized. Washington has imposed sanctions against Iran with the aim of halting all Iranian oil exports. European countries do not have sanctions against Iran, but have had them in place against Iran’s ally Syria since 2011.

Police in Gibraltar said on Thursday they had arrested the captain and chief officer of the supertanker and seized documents and electronic devices. Picardo said it was carrying 2.1 million barrels of light crude oil.

“All relevant decisions in respect of this matter were taken only as a direct result of the government of Gibraltar having reasonable grounds to believe the vessel was acting in breach of established EU sanctions against Syria,” Picardo said.

“We will not allow Gibraltar to be used or knowingly or unknowingly complicit in the breach of EU or other international sanctions.”

He said the provenance and origin of the cargo had not been relevant to the actions, which he also said could be challenged in the courts.