This handout picture provided by the office of Iran's Supreme Leader Ayatollah Ali Khamenei on Thursday shows Khamenei (right) greeting newly appointed commander of the Quds Force of the Islamic Revolutionary Guard Corps Esmail Qaani (left), Iranian Armed Forces Chief of Staff Major General Mohammad Bagheri (center), and Iranian Islamic Revolutionary Guard Corps Chief Commander Hossein Salami during a mourning ceremony in Tehran for slain top general Qasem Soleimani. (Photo: AFP)

The first of five Iranian tankers carrying fuel for gasoline-starved Venezuela entered the South American country's exclusive economic zone on Saturday, despite a US official's warning that Washington was considering a response to the shipment.

The tanker, named Fortune, reached the country's waters at around 7:40 pm local time (1140 GMT) after passing north of the neighboring dual-island Caribbean nation of Trinidad and Tobago, according to vessel tracking data from Refinitiv Eikon.

"The ships from the fraternal Islamic Republic of Iran are now in our exclusive economic zone," tweeted Tareck el Aissami, Venezuela's vice president of the economy and recently named oil minister.

Venezuelan state television showed images of a navy ship and aircraft preparing to meet the Iranian tanker. 

The defense minister had pledged that the military would escort the tankers once they reached Venezuela's Exclusive Economic Zone (EEZ) due to what authorities described as threats from the US.

The Iranian tanker flotilla is carrying a total of 1.53 million barrels of gasoline and alkylate to Venezuela, according to both governments, sources and calculations by TankerTrackers.com.

The desperately needed shipments have caused a diplomatic standoff between the US and Iran and Venezuela, which are under US sanctions. 

Gasoline is scarce in Venezuela due to a near-complete breakdown of the OPEC nation's 1.3 million barrel-per-day refining network.

Washington is considering measures in response, a senior US official said without elaborating further.

The US recently beefed up its naval presence in the Caribbean for what it called an expanded anti-drug operation. 

A Pentagon spokesman said on Thursday he was unaware of any operations related to the Iranian cargoes.

The shipment has drawn condemnation from Venezuela's opposition, which is concerned about growing ties between Iran and Venezuelan President Nicolas Maduro, who has overseen a six-year economic crisis. 

The shipments are bringing enough fuel for just a month of consumption at current rates in the nation, once a prominent fuel exporter.