Police in the British territory of Gibraltar on Friday released all four crew members of an Iranian tanker that was detained on suspicion of illegally transporting oil to Syria.

The crew of the Grace 1 were bailed without charge, and their release is subject to conditions, Royal Gibraltar Police wrote on Twitter. Officers had arrested the ship's captain and chief officer on Thursday and two second officers earlier on Friday.

The probe into whether the ship had breached the EU sanctions on Syria was still ongoing, police added.

British marines boarded the Grace 1 on July 4 as it passed through the Strait of Gibraltar to enter the Mediterranean Sea.

Iran denounced what it said was an "illegal interception" and Iranian President Hassan Rouhani warned the UK of unspecified "consequences" if it did not release the tanker and its crew.

On Thursday, the UK said three Iranian gunboats tried to stop and divert a British supertanker in the Strait of Hormuz, forcing naval frigate HMS Montrose to intervene to warn the ships away. Iran denied the incident took place.

Also read: Iran Warns US to Drop Sanctions as Time Runs out for Nuclear Deal

The UK said on Friday that it was sending a second navy ship, the HMS Duncan, to the oil-rich Persian Gulf as part of a pre-planned rotation to eventually relieve the HMS Montrose. A government source told the AFP news agency that the deployment had been sped up by several days.

The UK also said it was raising its threat level for ships travelling through Iranian waters to "critical".

Tensions are high in the Persian Gulf following separate attacks on several tankers that the US has blamed on Iran. The oil-rich country has denied any involvement.

US President Donald Trump said on Friday that Iran was "treading on very dangerous territory."

"Iran, if you're listening, you better be careful."

This article was originally published on Deutsche Welle.

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