The "first of several shipments" of Iranian fuel oil sets sail for Lebanon on Thursday, Hezbollah leader Hassan Nasrallah said.
Lebanon is in the midst of a severe fuel shortage that has caused long queues at petrol stations and power cuts for as much as 22 hours a day.
Iran has been under US sanctions for decades. Countries and individuals importing fuel from Tehran could also face sanctions.
“The first ship which will sail from Iran will be loaded with the needed commodities. All arrangements are finalised. It will set sail within hours to Lebanon. It will be followed by others,” Mr Nasrallah said.
Later on Thursday, Iran's semi-official Nournews reported that a group of Lebanese Shiite businessmen bought the fuel.
"The shipments are considered their property from the moment of loading” said the news website, which is close to Iran's Supreme National Security Council.
Lebanon's economic collapse caused the shortage of fuel and other essential imported goods.
The leader of the Iran-backed group announced the shipment in a speech to mark the occasion of Ashura, the most important religious holiday for Muslim Shiites.
During Ashura Shiites commemorate the death of Imam Hussein, the grandson of the Prophet Mohammed, at the Battle of Karbala in present-day Iraq in the 7th century.
The speech was broadcast live to thousands of supporters on giant screens in Hezbollah's stronghold in Beirut's southern suburbs.
“The first ship to Lebanon will carry fuel oil because it is vital to hospitals... and we tell the Americans and Israelis that the ship will be considered as Lebanese territory from the moment it sets sail,” he told the cheering crowds.
A Hezbollah spokesperson told The National the ship would take at least 10 to 15 days to reach Beirut
Mr Nasrallah announced the departure of the ship "to reassure people", she said. "Any attack on the ship will be considered an attack on Lebanese soil."
Lebanon's three-time prime minister Saad Hariri, who has close ties to Iran's regional rival Saudi Arabia, accused Hezbollah of treating Lebanon like an Iranian province.
The Shiite group was founded by Iran's Revolutionary Guard in 1982 and has been funded and armed by them since then. Hezbollah is the only Lebanese militia that was allowed to keep its weapons arsenal after Lebanon's civil war ended in 1990 and holds great sway over politics.
Mr Hariri also accused Iran of interfering in Lebanon's government formation, a process that has dragged on for more than a year as political leaders bicker over their share of ministerial portfolios.
"These decisions will double people's economic misery and pave a highway to hell," Mr Hariri said.
He added that Iranian ships will bring sanctions to the country, similar to the US sanctions on Venezuela, which has also received shipments of Iranian fuel.
Lebanon’s economy has been collapsing for the past two years as political leaders fail to implement the reforms needed to access billions of dollars in debt relief and loans from international lenders.
The central bank's dwindling foreign currency reserves have impeded imports of essential goods such as fuel and medicine, which are subsidised.
The fuel crisis has hit hospitals, bakeries and other essential services which say they could be forced to close.
A spokesperson for Lebanese President Michel Aoun said Tehran had yet to inform Beirut of the arrival of a fuel oil tanker.
“We have not taken any practical steps, Lebanon has not received any formal information from Iranian authorities about any fuel ship,” he told The National.
Mr Nasrallah said the logistics for fuel shipments from Iran were finalised. He mentioned last week that Hezbollah could bring in the fuel while bypassing the central bank to avoid breaching US sanctions.
He said Iran assured him it would stand by Lebanon and blamed the Lebanese government and American influence for the lack of response to the Iranian offer.
Mr Aoun's office said on Thursday that the US had granted Lebanon a waiver from possible sanctions to allow it to get electricity from Jordan via Syria.
Doing business with the Syrian regime, or individuals linked to it, is punishable under the Caesar Act.
“It took time but the Americans told us electricity can be supplied to Lebanon through Syria and this will not be considered a violation of the siege on Syria,” the president’s spokesperson said.
Washington will help provide Egyptian gas to Jordan to produce electricity, which will be channeled to Lebanon via regime-held Syria, the president’s statement said.
The US will also “facilitate the transfer of Egyptian gas through Jordan and Syria to northern Lebanon”.
The US embassy in Lebanon declined to comment.