Iranian attacks on vessels in and around the Strait of Hormuz reached their highest daily level since the war with the US began on Feb. 28, with six ships targeted in a single day, according to data from the UK Maritime Trade Operations agency (UKMTO) and international maritime security sources.

More than 18 commercial vessels were targeted by Iran during the first 12 days of October, including six on Monday, the data showed, The Caspian Post reports, citing Anadolu Agency.

Between Oct. 5 and 12 alone, at least 14 attacks, attempted attacks and interceptions involving vessels were recorded in and around the strait, according to the same sources.

UKMTO data and statements by Iran’s Islamic Revolutionary Guard Corps (IRGC) indicated a marked increase in attacks on commercial shipping since clashes between Tehran and Washington resumed in July.

At least 12 attacks on oil tankers and cargo ships were recorded in July, while at least 10 vessels were targeted in August, including oil and liquefied natural gas tankers off the UAE port of Fujairah and Oman’s Sohar.

The number rose to 18 vessels in September as the US and Gulf countries increasingly resorted to ship-to-ship transfers, using smaller vessels to transport oil before transferring it to larger tankers in safer areas.

The IRGC described the operations as targeting “illegal oil smuggling and an attempt to break the Hormuz blockade” and expanded its activities to include supply vessels, tugboats and small tankers in the Gulf.

The IRGC also claimed that some targeted vessels were damaged by explosions after striking naval mines while sailing “illegally” through routes it considered “unsafe.”

Tehran says its escalating operations against shipping are a response to a “US naval blockade.” It has also warned vessels against using routes it has not approved or entering the area after switching off their navigation and identification systems.

Since April, Iran has imposed restrictions on maritime traffic through the Strait of Hormuz, including designated transit routes and a requirement for prior coordination with the IRGC Navy.

On May 30, Tehran announced that vessels passing through the strait would have to obtain prior authorization from the IRGC Navy, warning that transit without permission would constitute a “security threat.”

The developments come amid continued tensions between Iran and the US over navigation through the Strait of Hormuz, one of the world’s most important strategic routes for energy and commodity supplies and a key issue in negotiations between the two sides.

The war began with US and Israeli attacks on Iran on Feb. 28, before a ceasefire was reached in April. Clashes between Washington and Tehran later resumed as negotiations stalled, despite the two sides signing a memorandum of understanding in June.