Hezbollah chief Hasan Nasrallah said Thursday a tanker would set off from Iran "within hours" to bring desperately needed fuel supplies to Lebanon, in defiance of US sanctions.

Many questions remain about how the shipment would reach its stated destination, where acute and growing fuel shortages have forced hospitals, businesses and government offices shut amid a crippling economic crisis.

But the move, prohibited by US sanctions on Iran's oil industry, could drag Lebanon into the covert naval war between Tehran and Israel. Nasrallah dared Iran's foes to stop the shipment.

"The vessel, from the moment it sails in the coming hours until it enters (Mediterranean) waters, will be considered Lebanese territory," he said during a televised speech to mark the Shiite Muslim commemoration of Ashura.

"To the Americans and Israelis, I say: it's Lebanese territory."

He said a first ship would bring fuel for "hospitals, manufacturers of medicine and food, as well as bakeries and private generators."

He said more vessels would follow to address shortages that have ground Lebanon to a halt.

Neither the Iranian nor the Lebanese governments have confirmed the paramilitary organisation's claim.

Nasrallah did not specify where or how the shipment would reach Lebanon and be offloaded.

Lebanese energy expert Laury Haytayan said major questions hung over the shipment, including the amount to be delivered, who would pay, where the boat would dock and whether the details of the transaction had been disclosed to the Lebanese government.

"It is a possibility that these tankers will go to Syria and shipments will be refined there," Haytayan said.

"But this is all prohibited by sanctions, its not that easy, and since Hezbollah is doing it in public, there is a lot of danger on Lebanon, we are in danger of being sanctioned, or being attacked."

Hezbollah chief Hassan Nasrallah has turned to his movement's ally Iran to ease Lebanon's crippling fuel shortages
Hezbollah chief Hassan Nasrallah has turned to his movement's ally Iran to ease Lebanon's crippling fuel shortages Al-Manar / -

Since February this year, Iran and Israel have been engaged in a "shadow war" in which vessels linked to each nation have come under attack in waters around the Gulf in tit-for-tat exchanges.

Hezbollah, designated as a terrorist group by much of the West, is a major political force in Lebanon and is the only group to have kept its arsenal of weapons following the end of the country's 1975-1990 civil war.

Hezbollah now has a more powerful arsenal than the Lebanese national army.

Former prime minister Saad Hariri warned Thursday that Nasrallah's remarks could be "dangerous."

"Iranian vessels will carry additional dangers and sanctions for the Lebanese," he said in a statement released by his office.

Lebanon is grappling with an economic crisis branded by the World Bank as one of the planet's worse since the mid-19th century.

The bankrupt state can no longer afford key imports nor subsidise essential goods, leading to crippling and sometimes deadly shortages of electricity, petrol and medicines among other things.

International donors have pledged hundreds of millions of dollars in assistance but have conditioned it on the creation of a cabinet capable of spearheading reforms.

But Lebanon's bitterly divided political leaders have repeatedly failed to agree on a new government a year after the previous one resigned in the wake of a monster blast in August at Beirut port.

Hariri on Thursday accused Iran of blocking the formation of a government, something Nasrallah dismissed.