Iran is escalating the war by attacking ships along a key oil route. Here’s what we know.

By Issy Ronald, CNN

(CNN) — Iran might be outgunned and outspent by the US and Israel, but it has one major advantage — its control over the Strait of Hormuz.

By attacking ships navigating the narrow waterway, Iran has effectively shut the route through which one-fifth of the world’s oil supply passes, bestowing the country with an outsized power compared to its diminishing military capabilities. Six vessels were struck in the Persian Gulf on Wednesday and Thursday, according to the UK’s maritime agency. Tehran has also reportedly mined the strait, further deterring ships from attempting passage and marking a new escalation in the war.

Iran’s new Supreme Leader Mojtaba Khamenei gave no sign of relenting on Thursday, warning in his first message that the strait will remain closed as a “tool of pressure.”

Here’s what you need to know:

What is Iran doing?

Iranian activity near the strait has become more aggressive in recent days as it attempts to choke off one of the global economy’s key arteries.

On Tuesday it emerged that Iran had begun laying mines in the strait, according to two people familiar with US intelligence on the matter.

The mining is not extensive, but Iran still maintains upward of 80% to 90% of its small boats and minelayers, one source told CNN, contradicting US President Donald Trump’s claim that Tehran has “no navy.”

Such a move underlines Iran’s reliance on asymmetric warfare and the havoc these tactics could wreak even as the country is battered by US-Israeli airstrikes.

Trump later sowed confusion about the issue, saying he didn’t think Iran had managed to lay any mines and that the US had taken out “just about all” their minelaying ships.

United Kingdom Maritime Trade Operations (UKMTO), a British organization run by the Royal Navy that provides security information to stakeholders, also cautioned that “there remains no confirmed evidence of mine deployment or detonation” in the waterway.

Iran has around 5,000 to 6,000 naval mines, a report from the US Congress estimated last year. That total includes many different types of mines, the report said. Some are limpet mines, which are usually manually attached to a ship’s hull by a diver; some are moored mines, which float just below the surface and explode when they come into contact with a ship; and some are “bottom” mines, which rest on the sea floor before detonating when they detect a nearby vessel.

Still, mines are just one part of the threat posed by Iran in the strait.

Its Islamic Revolutionary Guard Corps (IRGC), a military force that has its own navy, is still capable of deploying a “gauntlet” of mines, explosive-laden suicide boats and shore-based missile batteries, causing one US source to describe the strait as “Death Valley.”

Despite 12 days of airstrikes, the IRGC’s “main small craft fleet has largely remained intact,” said Farzin Nadimi, a senior fellow at the Washington Institute who has written about Iran’s naval strategy.

That has been laid bare over the last two days as six vessels have come under attack near the strait, according to the UKMTO, adding to the 10 others struck since the war began.

The IRGC claimed responsibility for several of these attacks, including saying they fired on the Thai-flagged bulk carrier Mayuree Naree as it tried to pass through, causing an explosion in the ship’s engine room. Three of the 23 crew members, who were believed to have been on duty in the engine room, remain unaccounted for, according to Thai authorities.

Two foreign oil tankers in Iraqi waters were set ablaze overnight Wednesday, killing at least one person. Iran said an underwater drone caused the explosions.

How has the US responded?

Securing the strait for commercial shipping is causing another headache for the US. It has responded by targeting Iranian minelayers and is looking at escorting ships, though Energy Secretary Chris Wright has said the US Navy is not yet ready to do so.

US Central Command said Tuesday it destroyed multiple Iranian naval ships — including 16 minelayers — near the strait, though it did not mention destroying any mines.

American minesweeping capabilities in the Persian Gulf are not as strong as they once were. The US Navy decommissioned the last of its four dedicated minesweepers in the region last September, leaving it dependent on less specialized ships.

At the time, US Naval Forces Central Command said four littoral combat ships (LCS) would assume responsibility for minesweeping. These vessels have a history of problems, however, leading their critics to dub them “Little Crappy Ships.”

Trump has proposed providing escorts for shipping in the strait, but this would put naval vessels in harm’s way with no obvious strategic advantage for the war itself.

While top US general Dan Caine said the US would look at a “range of options” if tasked with escorting vessels, Wright noted that the navy would likely only be capable of this by the end of the month.

While it’s technically possible to create the conditions for some ships to pass through the strait within days or weeks, restoring long-term sustained security to the strait could take months, according to Nadimi, the analyst. “It cannot be done quickly and it cannot be done under enemy fire,” he said.

Even if escorts can be managed, oil tanker flows would likely remain at least 10% less than normal, according to Lloyd’s List Intelligence. That figure could be “significantly lower depending on Iran’s response,” and any potential minesweeping operations, it said.

What’s at stake?

The longer the Strait of Hormuz remains impassable, the graver the implications for the global economy.

With the strait effectively closed, 15 million barrels of crude and 5 million barrels of other oil produces remain stranded in the Gulf, according to the director of the International Energy Agency.

Even in peacetime, great skill is required to navigate the strait’s narrow channel and busy traffic. These threats add to the dangers for any vessel trying to pass and make opening it up again more difficult.

“They want to inflict pain,” Nadimi told CNN. “The strategic objective is to inflict as much punishing pain as necessary on the US military bases in the region, Israeli homeland, Persian Gulf countries, and indirectly the US homeland.”

While there are some other options for exporting crude via pipelines, the world’s biggest oil exporter, Saudi Aramco, warned on Tuesday of the potentially “catastrophic consequences” for oil markets if flows don’t resume through the strait.

Even the IEA’s announcement it had agreed to release 400 million barrels of oil into the global market – the largest such emergency release in history – did little to quell oil market volatility. And those 400 million barrels only staunch the shortfall for 26 days.

Already in Asia, which imports 60% of its oil from the Middle East, some countries are taking drastic measures to reduce their energy consumption. Schools have closed in Pakistan, South Korea is imposing a fuel price cap for the first time in 30 years, and Thailand has ordered government employees to work remotely.

Has this happened before?

Yes. During the Iran-Iraq war in the 1980s, both countries targeted each other’s oil tankers in the Persian Gulf. Iran’s navy laid mines close to the Strait of Hormuz, and one of these struck a US warship, the USS Samuel B. Roberts.

The ship suffered substantial damage, and the Reagan administration retaliated by damaging or sinking three Iranian warships and three oil platforms, drastically reducing Tehran’s ability to engage in the gulf.

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CNN’s Natasha Bertrand, Brad Lendon, Catherine Nicholls, Tim Lister and Sophie Tanno contributed to this report.