Iran's Islamic Revolutionary Guard Corps (IRGC) said two oil tankers caught fire after hitting naval mines in the Strait of Hormuz, each carrying about two million barrels of crude oil.

The first vessel hit a mine, setting off an explosion and a "fierce fire", the IRGC said, while a second was engulfed in a "massive blaze", according to the official IRNA news agency. IRNA said both ships had switched off their positioning and radar systems and used a route Iran had not authorised. The IRGC claimed one tanker tried to cross the waterway "with US support".

There was no independent confirmation of the reports, and the names of the vessels and the origin of their cargo were not immediately known.

The incident adds to a sharp rise in attacks around the waterway. BBC Verify, citing data from the UK Maritime Trade Operations agency, said 12 vessels reported being attacked between 5 and 11 October - the largest weekly total since the US-Israeli war with Iran began in late February. Six of those ships were hit on 5 October alone, a one-day high.

Oil shipments through the strait have slowed as the strikes have mounted. About 12.6 million barrels a day passed through on 13 October, down from some 15 million at the end of September and a pre-war baseline of about 17 million, according to the trade intelligence firm Kpler.

Iran took control of the strait after the war began and now requires ships to seek its permission before transiting; the United States has imposed a naval counter-blockade on Iranian oil exports. This week the IRGC's naval commander, Rear Admiral Ali Azmaei, warned that any vessel that switched off its navigation systems would be treated as "hostile".

About a fifth of the world's oil passed through the Strait of Hormuz before the war. The disruption has pushed up global energy prices and inflation, with knock-on costs for importing countries such as Bangladesh.