(Bloomberg) -- Iran’s military vowed to retaliate against the of an oil tanker loaded with the Islamic Republic’s crude off the coast of Gibraltar last week.

“It will be reciprocated, at a suitable time and in a suitable place,” Mohammad Bagheri, the chief of staff for Iran’s armed forces, was quoted as saying by the official Islamic Republic News Agency.

The warning highlights mounting risks to shipping in a region that exports about a third of all seaborne petroleum. BP Plc is empty inside the Persian Gulf, close to Saudi Arabia, rather than risk its seizure by Iran, a person familiar with the matter said Monday. Six tankers have been attacked since early May, with the U.S. blaming Iran for the incidents, a charge Tehran denies.

The tanker seized by Gibraltar with the help of Royal Marines -- the Grace 1 -- was arrested on the grounds it was taking crude to a sanctioned Syrian company, according to the British overseas territory on Spain’s southern tip. Iran said the tanker was in international waters, , and described the U.K.’s actions as piracy.

Insurance costs soared in the aftermath of the tanker attacks and some owners initially became wary of sailing to the region.

Tensions escalated since the U.S. exited an international nuclear accord with Iran then resumed -- and escalated -- sanctions against nation. The Persian Gulf country has responded by suspending some of its commitments as part of the accord not to develop nuclear activities that it says are for are civilian purposes.

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Alaric Nightingale, Brian Wingfield