Iran, which feels threatened by the presence of U.S. bases and warships in the region, has warned for years that it would choke off the strait in case of war or economic sanctions. The narrow passage at the entrance to the Persian Gulf hosts a daily caravan of tankers that transport roughly a third of the world’s oil shipments.

The European Union, encouraged by the United States, is expected to decide in January whether to boycott Iranian crude. And countries such as Japan and South Korea are under increasing U.S. pressure to stop buying oil from Iran, currently the world’s fifth-largest producer.

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