Iran said Sunday that it had struck an unmanned U.S. vessel attempting to enter the Strait of Hormuz, marking the latest escalation in renewed fighting between the two countries.

The United States did not immediately confirm the reported attack. It came a day after the U.S. military said it had struck three Iranian oil tankers in response to a missile attack targeting U.S. Navy warships.

The conflict began with U.S. and Israeli attacks on Iran on Feb. 28. Although a ceasefire was announced in June, sporadic clashes have continued as negotiations between the two sides collapsed.

Fighting resumed last week after about a month of relative calm, with attacks again focusing on the Strait of Hormuz and Iranian communities along the strategic waterway. At least five people were killed earlier in the week during U.S. strikes in southern Iran.

The Trump administration appears to be pursuing both military and economic pressure, responding to attacks in the strait while targeting foreign financial institutions accused of helping Iran move money.

Despite years of sanctions and the current U.S. blockade, Iran has continued finding ways to export oil, relying partly on a so-called shadow fleet to transport crude to buyers and ease growing economic pressure.

Meanwhile, efforts to resolve disputes over Iran's nuclear programme have stalled after negotiations broke down shortly after the two sides signed a memorandum of understanding in mid-June.

Diplomats say the United States, Britain, France and Germany are seeking to refer Iran to the U.N. Security Council over Tehran's alleged failure to meet its nuclear nonproliferation obligations.

Iran, however, is increasingly using its position around the Strait of Hormuz as leverage. The waterway is a vital route for global oil and natural gas shipments and had been regarded as an international passage before the war began.