Iran dismissed as "baseless" on Friday US accusations it executed twin attacks that left two tankers ablaze in the Gulf of Oman, raising fears of conflict in the strategically vital waterway.

China called for all sides to resolve the conflict through dialogue as oil prices jumped, while the EU and the UN called for restraint. 

US Secretary of State Mike ­Pompeo warned that Washington would defend its regional interests after US Central Command blamed Iranian forces for the attacks - the second in a month in the strategic shipping lane.

The US Central Command released grainy black-and-white video it said showed crew members of an Iranian patrol boat removing an "unexploded limpet mine" from the hull of Japanese-owned tanker Kokuka Courageous. 

No proxy group could be responsible, Pompeo said, but Iranian Foreign Minister Mohammad Javad Zarif tweeted that the US had "immediately jumped to make allegations against Iran without a shred of factual or circumstantial evidence."

He accused Washington of seeking to "sabotage diplomacy" as Japanese Prime Minister Abe Shinzo visited Iran. One of the targeted vessels is owned by a Japanese company while the other was Norwegian-operated.

"It is a serious case which threatens our country's peace and stability," Japan's foreign ministry said in a statement, adding that the straits where the incidents occurred were "vital for our country's energy security."

With tensions spiraling between Iran and the US, UN chief Antonio Guterres warned against a Gulf confrontation.

But Iran labeled the attacks "suspicious," as its supreme leader Ayatollah Ali Khamenei rebuffed overtures by Abe to open talks with US President Donald Trump.

Iranian President Hassan Rouhani called the US a "serious threat to global stability" as he attended an international forum in Kyrgyzstan.

The two vessels, which were 10 nautical miles apart en route to Asia, were struck by explosions in the early daylight hours Thursday after passing through the Strait of Hormuz some 25 nautical miles off Iran's southern coast.

There was no claim of responsibility for the blasts, which struck both tankers at the waterline.

Oil prices jumped at the threat of open conflict around the Strait of Hormuz, the chokepoint between the Persian Gulf and the Gulf of Oman, through which some 15 million barrels of crude pass daily.