Global crude oil prices rose on Friday as growing military tensions involving Iran renewed concerns over supply disruptions. At 1:30 a.m. Greenwich Mean Time, Brent crude was up 9 cents, or 0.1%, at $87.16 a barrel, while West Texas Intermediate gained 4 cents to $81.29. Supply worries intensified after the United States threatened to maintain a naval blockade on Iran indefinitely.

The Strait of Hormuz remains central to market concerns. Hossein Taieb, head of Iran’s Basij paramilitary force, said the strategic waterway was under the Islamic Republic’s management and control, according to the semi-official Fars news agency. The UAE’s state news agency WAM said two Abu Dhabi National Oil Company vessels were attacked while passing through the strait on Thursday evening, and the UAE government blamed Iran.

Demand concerns are limiting the price increase. OPEC and the International Energy Agency lowered their forecasts for global oil-demand growth this week, while US crude inventories posted their largest weekly increase in more than three and a half years. KCM chief market analyst Tim Waterer said supply concerns supported prices, but weaker demand expectations prevented sharper gains.