President Trump then slapped unilateral trade and financial sanctions on Iran.
The US had initially provided South Korea with a waiver to avoid implementing the new sanctions, but those lapsed, essentially forcing Seoul to freeze the Iranian assets.
Iran's move to seize the vessel also coincides with Tehran's announcement that it will increase the level of uranium enrichment at its Fordo nuclear facility to 20%, well above that agreed to with several countries, including the US, until Trump's withdrawal from the deal in 2018. Uranium enriched to 90% is required to fuel nuclear weapons. Iran claims its nuclear program is strictly for peaceful purposes.