The Iranian Foreign Ministry warned of “heavy consequences” on Monday should the United States seize an oil tanker recently released by Gilbraltar after being impounded for six weeks.

The Adrian Darya 1 was impounded early last month by the British Royal Marines off the coast of Gibraltar over suspicions that it was transporting Iranian crude oil to Syria in breach of European Union sanctions against Bashar al-Assad’s regime. The ship, previously named the Grace 1, was transporting around 2.1 million barrels of oil worth an estimated $130 million.

Tehran has repeatedly denied claims from the United Kingdom that the oil was destined Syria, declaring the impoundment “illegal” and accusing the United Kingdom of “piracy.”

They soon retaliated by seizing the British-flagged oil tanker Stena Impero in a tit for tat move, heightening tensions between Iran and the West. The ship is still yet to be released, with Iranian Foreign Ministry spokesperson Abbas Mouzani disputing claims it was an act of retaliation.

“Regarding the release of the law-breaking British tanker [Stena Impero], we have to wait for the court’s ruling,” he said. “This tanker has committed two to three nautical violations that are being investigated. We hope that these investigations will finish as soon as possible and a verdict will be issued and if the verdict orders its release, it can continue to sail its path.”

Meanwhile, the Adrian Darya was released around 11 pm on Sunday evening after the Gibraltar Supreme Court rejected a last-ditch attempt by the United States to hold the ship, and is currently heading east through the Mediterranean Sea.

However, there is now speculation that U.S. naval forces may attempt to seize the ship, a move that Mousavi said would lead to “heavy consequences” for those involved.

“If such an action is taken or even if it is stated verbally and not done, it is considered a threat against the maritime security in international waters,” he told a news conference on Monday.

“The Islamic Republic of Iran has given necessary warnings to the US officials through official channels, especially the Swiss Embassy in Tehran, not to commit such a mistake because it would have grave consequences,” he continued.

Mousavi also claimed that the vessel’s release represented a blow to U.S. “unilateralism,” as the Trump administration leads international efforts to sanction and isolate Iran over their nuclear weapons program.

“The Americans have not been very successful with their unilateral sanctions that have no legal basis,” he said. “They should come to their senses that bullying and unilateralism cannot get anywhere in the world today.”

Heshmatollah Falahatpisheh, a senior Iranian legislator who sits on the country’s national security and foreign affairs committee, also warned that the diplomatic row was not over until Adrian Darya 1 reached its destination.

“Until the Iranian oil tanker arrives at its destination the British must help end the crisis,” he was quoted as saying by the semi-official ISNA news agency.”This means that the crisis with Britain is not over. Britain has the primary responsibility for ending the oil tanker crisis.”

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