Washington DC - President Donald Trump said Sunday that NATO faces a "very bad" future if members fail to help open the Strait of Hormuz, the critical oil transport waterway effectively shut by Iran as a response to the war waged by the US and Israel.

"If there's no response or if it's a negative response I think it will be very bad for the future of NATO," Trump told the Financial Times.

The 79-year-old has over the years criticized the alliance as freeloading at the expense of the US and said he expects Europe to help, given American aid to Ukraine.

Trump also said an upcoming summit in Beijing with his Chinese counterpart Xi Jinping could be delayed as he presses for China's help to open the strait.

"We'd like to know before" the summit, Trump said, noting that China as well as many European countries rely more than the US on oil flowing from the Gulf.

"It's only appropriate that people who are the beneficiaries of the Strait will help to make sure that nothing bad happens there," Trump said.

Trump said separately to reporters traveling with him on Air Force One that the US was in discussions with "about seven" countries on getting help to reopen the Strait of Hormuz. His call on Saturday for other states to "send ships" has been met muted response so far.

Asked about specific help he was looking for, Trump told the FT he wanted minesweepers as well as "people who are going to knock out some bad actors that are along the (Iranian) shore."

Uncertainty about how long the war against Iran might last has rattled oil markets, where crude prices have surged over the past two weeks due to the supply risks.

On Sunday, the US benchmark West Texas Intermediate opened 2.5% higher at $100.22 a barrel, while the price of Brent, the international benchmark, rose 2.9% to $106.11 a barrel.