TEHRAN, Iran (AP) — Iran’s paramilitary Revolutionary Guard launched a surprise military drill Wednesday on disputed islands in the Persian Gulf, just as the U.S. military increase its presence in the region over recent ship seizures by Tehran.

The drill focused primarily on Abu Musa Island, though the Guard also landed forces on the Greater Tunb Island as well, Iran’s state-run IRNA news agency reported. Ships, drones and missile units took part in the drill, the report said.

Iran did not provide a reason for launching the drill, though such snap exercises have happened in the past.

“We always try for security and tranquility; it is our way,” the Guard’s chief, Gen. Hossein Salami, said in a televised address during the drill. “Our nation is vigilant, and it gives harsh responses to all threats, complicated seditions and secret scenarios and hostilities,”

However, the drill comes as thousands of Marines and sailors on both the amphibious assault ship USS Bataan and the USS Carter Hall, a landing ship, are on their way to the Persian Gulf. Already, the U.S. has sent A-10 Thunderbolt II warplanes, F-16 and F-35 fighters, as well as the destroyer USS Thomas Hudner, to the region.

The Pentagon has said the deployment is “in response to recent attempts by Iran to threaten the free flow of commerce in the Strait of Hormuz and its surrounding waters.” Some 20% of the world’s oil passes through the narrow waterway connecting the Persian Gulf to the wider world and the U.S. views it as crucial to both its national security and keeping global energy prices stable.

Meanwhile, Iran now enriches uranium closer than ever to weapon-grade levels after the collapse of its 2015 nuclear deal with world powers.

The use of Abu Musa and Greater Tunb in the drill also provides another message to the region. Those two islands remain claimed by the United Arab Emirates, home to Abu Dhabi and Dubai. Iran’s late shah seized the islands in 1971 just before the UAE became an independent country and Tehran has held the islands since. Lesser Tunb Island was also seized.

Seizing those islands reminds Iran’s neighbors of its military might as Tehran’s diplomats have been trying to convince Gulf Arab countries allied with the U.S. that “foreigners” aren’t needed to secure the region.

Meanwhile, Iran has been trying to signal its displeasure over recent comments about the islands made by Russia, which Tehran has supplied with bomb-carrying drones for their war in Ukraine. Russia earlier this summer in a joint statement with the Gulf Cooperation Council called for “bilateral negotiations or the International Court of Justice” to decide who should control the islands. That prompted an outcry in Iran and Tehran summoned the Russian envoy over the remarks.

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Gambrell reported from Dubai, United Arab Emirates.