Iran said Sunday it struck an unmanned U.S. vessel trying to enter the Strait of Hormuz, the latest salvo in a new flare-up of fighting between the two countries.

The U.S. did not immediately confirm the purported attack, which came a day after the U.S. military said forces struck three Iranian oil tankers in response to a missile attack on Navy warships.

The war began with U.S. and Israeli attacks on Iran on Feb. 28. But since a ceasefire agreement was announced in June, on-and-off fighting has persisted with both sides trying to inflict military and economic pain as negotiations have collapsed.