Iran's President Hassan Rouhani said on Wednesday Britain would face "consequences" over the seizure of an Iranian oil tanker.
Iran has demanded the immediate release of the Grace 1, which British Royal Marines boarded off Gibraltar last week and seized on suspicion that it was breaking sanctions by taking oil to Syria.
"You (Britain) are the initiator of insecurity and you will realize the consequences later," Rouhani said after a cabinet meeting, in remarks carried by state television.
"Now you are so hopeless that, when one of your tankers wants to move in the region, you have to bring your frigates (to escort it) because you are scared. Then why do you commit such acts (seizure)? You should instead allow navigation to be safe," Rouhani said.
Rouhani also said Iran's decision to increase uranium enrichment would produce fuel for power plants and serve other peaceful aims, and that it was within the framework of Iran's 2015 nuclear deal with world powers.
He was speaking a day after European powers accused Iran of "pursuing activities inconsistent with its commitments" under the nuclear deal and called for an urgent meeting of the parties to the agreement.
"You (Europeans) do not need to worry about Iran, your concern must be over the United States, which has violated this whole commitment and undermined international obligations," Rouhani said.
Britain, Germany, France, Russia, China and Iran are the remaining parties to the deal, which the United States abandoned last year