The Islamic Revolutionary Guard Corps held naval drills involving hundreds of boats in the Persian Gulf, just as a South Korean delegation headed to Tehran to negotiate the release of a tanker seized by Iran earlier this week.

The IRGC’s volunteer Basij paramilitary force led a parade of more than 700 light and mid-weight vessels in the waters off Assalouyeh on Thursday, around 450 kilometers (280 miles) west of the port of Bandar Abbas where the Korean ship is being held, according to a statement on the IRGC’s news website, Sepah News.

The IRGC’s naval fleet is mostly comprised of small, agile boats that it uses to police shipping routes around the Strait of Hormuz. On Monday some of them seized the South Korean-flagged Hankuk Chemi for alleged violations of environmental laws, something its operator has denied.

Ship Seizure Drags South Korea Deeper Into Tensions Over Iran

South Korea has already sent its own warship, the ROKS Choi Young, toward waters near the strait, and a diplomatic delegation left for Iran early on Thursday to negotiate the tanker’s release, Yonhap news agency reported.

Iran has used its influence over the Persian Gulf to push back against U.S.-led attempts to isolate it economically. A government spokesman this week linked the seizure of the tanker to billions of dollars trapped in South Korea by U.S. sanctions.

Thursday’s parade was part of commemoration events for the one-year anniversary of the killing of IRGC commander Qassem Soleimani by the U.S., according to the statement.

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