President Trump's pick to become the military’s next top officer says Iran has raised its “intensity of malign activity” since the U.S. pulled out of a landmark 2015 nuclear deal.

Army Chief of Staff Gen. Mark Milley’s comments Thursday come following the Islamic Republic’s harassment of a British oil tanker yesterday in the Strait of Hormuz. Five Iranian Islamic Revolutionary Guard Corps gunboats tried to seize the vessel but backed off after a British warship approached, a senior U.S. defense official told Fox News.

Milley, speaking to lawmakers at the Pentagon Thursday, said “their intensity of malign activity I think has increased” after the Trump administration’s withdrawal from the nuclear pact.

“They have been a malign actor for many, many years,” he added.

Milley also revealed that he supports the formation of a new coalition to escort vessels in the Persian Gulf. Britain raised its threat level for ships there this week to its highest point – one day before their oil tanker was harassed in the Strait of Hormuz.

The goal is to have U.S. military vessels serve as “command ships” and have other nations in the region escort the tankers through the volatile Strait of Hormuz, like we saw with the British warship escorting the UK-flagged tankers.

According to Lloyd’s List, a leading shipping news outlet, there are currently seven UK or Isle of Man-flagged vessels in the Persian Gulf region.

IRANIAN BOATS ATTEMPT TO SEIZE BRITISH OIL TANKER

Last month near the Persian Gulf, Iran shot down a U.S. drone over the Strait of Hormuz, a vital waterway separating Iran from the United Arab Emirates. U.S. officials, in addition, have blamed Iran for recent attacks on six oil tankers in the adjacent Gulf of Oman.

In Gibraltar Thursday, police arrested the captain and chief officer of an Iranian supertanker that officials believed was operating in violation of European Union sanctions, Reuters reported. The British Royal Marines seized the vessel there last week amid suspicions that it was trying to deliver crude oil to Syria, an ally of Iran.

Iranian President Hassan Rouhani has warned that Britain would face repercussions over the seizure.

Iranian President Hassan Rouhani speaks during a cabinet meeting in Tehran, Iran, on July 3. (AP/Iranian Presidency Office)

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In the oil tanker incident Wednesday, the British warship that came to the rescue intercepted the Iranian boats and threatened to open fire. A manned U.S. reconnaissance aircraft was above as well, a senior U.S. defense official said, adding that Iranian forces left without opening fire.

“Threats to international freedom of navigation require an international solution,” said Navy Captain Bill Urban, spokesman for the U.S. Central Command. “The world economy depends on the free flow of commerce, and it is incumbent on all nations to protect and preserve this lynchpin of global prosperity.”

Fox News’ Lucas Tomlinson and Louis Casiano contributed to this report.