U.S. Secretary of State Mike Pompeo said on Sunday that he was "very confident" the U.S. would be able to build a maritime coalition in the Gulf, despite a lukewarm response from European and Asian allies.

Pompeo and U.S. Defense Secretary Mark Esper were speaking alongside their Australian counterparts in Sydney.

On the same day, Iran's Islamic Revolutionary Guards Crops (IRGC) seized an Iraqi oil tanker in the Gulf which they said was smuggling fuel and detained seven crewmen, reported Iranian state media.

The vessel was intercepted near Iran's Farsi Island in the Gulf, Iran's semi-official Fars news agency said.

The elite IRGC has a navy base on Farsi Island which is located north of the Strait of Hormuz.

"The IRGC's naval forces have seized a foreign oil tanker in the Persian Gulf that was smuggling fuel for some Arab countries," the Guards commander Ramezan Zirahi told state TV.

The state news agency IRNA said it was an Iraqi ship that was seized on Wednesday night in the Gulf, quoting the Guards.

Zirahi said it was carrying 700,000 liters of fuel, without elaborating on the nationalities of the detained crewmen.

"The boats of the IRGC navy were patrolling the area to control traffic and detect illicit trade when they seized the tanker," news agency Fars quoted Zirahi as saying, adding that the seizure was in coordination with Iran's judicial authorities.

Iraq's oil ministry, however, denied any ties to the oil tanker seized by Iran in a statement on Sunday, also to the fuel-smuggling accusation.

"The ministry does not export diesel to the international market," reported Iraqi News Agency.

Iran, which has some of the world's cheapest fuel prices due to heavy state subsidies and the fall of its currency, has been fighting rampant fuel smuggling by land to neighboring countries and by sea to Gulf Arab states.

"The tanker was transferred to the Bushehr port, where its fuel was handed over to the authorities," Zirahi added.

A spokesman for the U.S. Navy's Bahrain-based Fifth Fleet said they had no information to confirm Iran's media reports.

Iranian English-language Press TV aired a video that it said "shows the process through which the IRGC have halted the ship and taken it into custody."

"The ship was seized in Iranian territorial waters and had been transporting diesel fuel," Press TV added.

Another oil tanker, the Panama-flagged MT Riah, was captured by the elite force last month for allegedly smuggling fuel.

Tensions have risen between Iran and the West since last year when the U.S. pulled out of an international agreement which curbed the Islamic Republic's nuclear program in return for an easing of economic sanctions on Iran.

Fueling fears of a Middle East war with global repercussions, the Guards seized British tanker Stena Impero near the Strait of Hormuz in July for alleged marine violations, two weeks after British forces captured an Iranian oil tanker Grace 1 near Gibraltar accused of violating sanctions on Syria.

Angered by intensified U.S. sanctions designed to strangle its vital oil trade and the failure of Britain and European parties to the agreement to salvage the pact, Tehran has decreased its commitments to the nuclear deal.

Iran also has threatened to block all exports through the Strait, if countries heed U.S. calls to stop buying Iranian oil. One-fifth of global oil consumption passes through the Strait from Middle East crude producers to major markets.

"The Persian Gulf is like a tinderbox and explosion of a firecracker can lead to a huge disaster," the semi-official Mehr news agency quoted Guards commander Brigadier General Ahmadreza Pourdastan as saying on Sunday.

"All countries which have interests in the region absolutely are not willing to see a new regional crisis," Brigadier General added.