Tensions between Iran and the United States have escalated over the past 18 months, with a series of incidents in the Gulf, the latest involving a South Korean tanker seized by Tehran.

Here is a recap:

On May 5, 2019, the US announces the deployment of an aircraft carrier and a bomber task force to the Middle East, bringing a military dimension to its standoff with arch-rival Iran.

The move comes almost exactly a year after Washington unilaterally withdrew from an international 2015 deal curbing Iran's nuclear programme and reimposed sanctions.

On May 12, four ships, including three oil tankers, are damaged in mysterious attacks in the Gulf that the US and Saudi Arabia blame on Iran.

Two tankers, Norwegian and Japanese, come under attack in the Gulf of Oman on June 13. Washington, London and Riyadh blame Iran, which denies involvement.

Iran's Revolutionary Guards say on June 20 they shot down a US drone that violated Iranian airspace near the Strait of Hormuz, a crucial transit point in the Gulf for world oil exports.

The Pentagon confirms the downing but says the unmanned aircraft was in international airspace.

US President Donald Trump approves a retaliatory strike, but cancels it at the last minute.

On July 18, Trump says the US military has taken down an Iranian drone that came dangerously close to one of its naval vessels in the Strait of Hormuz.

Tensions are exacerbated by Iran's seizure of three foreign tankers, one of which is British-flagged, a move seen as retaliation for Britain impounding an Iranian tanker off Gibraltar in early July.

On September 14, aerial attacks claimed by Yemen's Iran-backed Huthi rebels spark fires at two major Saudi oil facilities.

Tehran is accused of being responsible but denies involvement.

On December 31, pro-Iran protesters storm the US embassy compound in Baghdad, expressing fury over US strikes against bases of a pro-Iran group in Iraq.

Three days later a US strike kills top Iranian commander Qasem Soleimani, the commander of the Quds Force, the Revolutionary Guards' foreign arm, in Iraq.

The Pentagon says Trump ordered the killing after the siege of the US embassy.

Iran in retaliation launches a volley of missiles at Iraqi bases housing US and other coalition troops on January 8, 2020.

The same day, Iran shoots down in "error" a Ukrainian passenger jet shortly after take off from Tehran, killing all 176 people on board.

On April 15, the Pentagon accuses Revolutionary Guards boats of dangerous and provocative actions after its vessels harass US ships in the Gulf.

A week later Iran says it has put its first military satellite into orbit.

On November 27, one of Iran's most prominent nuclear scientists Mohsen Fakhrizadeh is killed near Tehran in an attack on his convoy.

Iran accuses Israel of having commanded the attack on the scientist, who is later described as a deputy defence minister. Israel does not comment on the accusations.

On January 4, 2021, the Revolutionary Guards seize a South Korean-flagged tanker in the Gulf for breaking "environmental laws".

On the same day Iran says it has started the process to enrich uranium to 20 percent purity, well beyond the threshold set by the 2015 nuclear deal.

The Pentagon reverses a decision to bring the USS Nimitz aircraft carrier home from the Gulf.

While demanding the release of its ship, South Korea sends a destroyer carrying members of its anti-piracy unit towards the Strait of Hormuz. Seoul then says it will send a delegation to Iran to negotiate.

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