What to know about the Strait of Hormuz, which Iran says it temporarily closed for a military drill

In past times of tension and conflict, Iran has at times harassed shipping though the narrows, and during the 1980s' Iran-Iraq war, both sides attacked tankers and other vessels, using naval mines to completely shut down traffic at points. But Iran has not carried out repeated threats to close the waterway altogether since the 1980s, even during last year's 12-day war when Israel and the U.S. bombarded Iran's key nuclear and military sites.

The extent and impact of Tuesday's closure were not immediately known. Iranian media said it would be for several hours for “safety and maritime concerns.”

The U.S. military’s Central Command did not immediately comment on the closure or Iran's live fire drills. But during Iranian military exercises in the strait and nearby waters several weeks ago, it warned Tehran that any “unsafe and unprofessional behavior near U.S. forces, regional partners or commercial vessels increases risks of collision, escalation and destabilization.”

Here’s what to know about the strait, the drill, what caused the tensions and what might happen next.

A key waterway for global shipping

The Strait of Hormuz is a bending waterway, about 33 kilometers (21 miles) wide at its narrowest point. It connects the Persian Gulf to the Gulf of Oman. From there, ships can then travel to the rest of the world. While Iran and Oman have their territorial waters in the strait, it's viewed as an international waterway all ships can ply. The United Arab Emirates, home to the skyscraper-studded city of Dubai, also sits near the waterway.

The strait long has been important for trade

The Strait of Hormuz through history has been important for trade, with ceramics, ivory, silk and textiles moving from China through the region. In the modern era, it is the route for supertankers carrying oil and gas from Saudi Arabia, Kuwait, Iraq, Qatar, Bahrain, the UAE and Iran. The vast majority of it goes to markets in Asia, including Iran's only remaining oil customer, China.

While there are pipelines in Saudi Arabia and the UAE that can avoid the passage, the U.S. Energy Information Administration says “most volumes that transit the strait have no alternative means of exiting the region.”

Threats to the route have spiked global energy prices in the past, including during the Israel-Iran war in June.

Iran drill includes live fire

With the U.S. threatening to strike amid the massive nationwide anti-government protests that erupted in late December and January, Iran held a live-fire military drill in the Strait of Hormuz in early February. It warned ships of the drill at the time but did close the passage.

On Feb. 4, tensions between the Iranian and U.S. navies rose further after a U.S. Navy fighter jet shot down an Iranian drone that was approaching the aircraft carrier USS Abraham Lincoln in the Arabian Sea. Iran also harassed a U.S.-flagged and U.S.-crewed merchant vessel that was sailing in the Strait of Hormuz, the U.S. military reported.

On Monday, Iran announced its new drill, dubbed, “Smart Control of the Strait of Hormuz” military drill. Mariners in the region were warned by radio that it planned “live surface firing.”

The semiofficial Tasnim news agency, which is close to the country’s paramilitary Revolutionary Guard, reported a live missile test Tuesday morning, saying missiles launched from inside Iran and along its coast had struck their targets in the Strait of Hormuz.

US CENTCOM has previously said Iran has a “right to operate professionally in international airspace and waters,” but it warned against interfering or threatening American warships or passing commercial vessels. The command, which oversees the U.S. Navy’s Bahrain-based 5th Fleet, said it would not tolerate actions such as Iranian aircraft or vessels getting too close to American warships or pointing weapons toward them.

Iran's Supreme leader issues sharp threat to U.S.

The actions around the strait come amid increasing tension between the U.S. and Iran. U.S. President Donald Trump initially threatened to launch a military strike against Iran after its bloody crackdown on last month’s protests. Since then, he has shifted to threatening attack to pressure Tehran to make a deal over its nuclear program. The two sides held a new round of indirect nuclear negotiations in Geneva on Tuesday.

The USS Abraham Lincoln aircraft carrier and supporting guided missile destroyers have been in the Arabian Sea for several weeks, where they could launch an attack if Trump calls for it. Trump said Friday the USS Gerald R. Ford, the world’s largest aircraft carrier, was being sent from the Caribbean to the Mideast to join other military assets the U.S. has built up in the region.

Iran has warned it could launch its own preemptive strike or target American interests across the Middle East and Israel. While the 12-day war saw Iran fire off ballistic missiles and Israel target its stockpile, Tehran maintains an arsenal of short- and medium-range missiles that could hit surrounding Gulf Arab states.

On Tuesday, Iran’s Supreme Leader Ayatollah Ali Khamenei warned the U.S that “the strongest army in the world might sometimes receive such a slap that it cannot get back on its feet.”

“Of course a warship is a dangerous apparatus, but more dangerous than the warship is the weapon that can sink the warship into the depths of the sea,” Khamenei said, Iranian state TV reported.

Associated Press writer Melanie Lidman contributed from Tel Aviv, Israel.