President Donald Trump has vowed to starve Cuba of oil after the US military ousting of Nicolas Maduro, the leader of Venezuela, which had been the communist nation's main supplier of the commodity.

Mexico's President Claudia Sheinbaum has protested against the humanitarian impact of Trump's threats to impose tariffs on any country sending crude to Cuba, and promised to provide aid.

The ships Papaloapan and Isla Holbox, sent by Sheinbaum's government, entered Havana Harbor on Thursday, an AFP team observed.

They are carrying fresh and powdered milk, meat, beans, rice and personal hygiene items, the Mexican foreign ministry says.

Trump and Secretary of State Marco Rubio, the Miami-born son of Cuban immigrants, have made no secret of their desire to bring about regime change in Havana.

The Republican leader has said Cuba is "ready to fall."

The island of 9.6 million inhabitants, under a US trade embargo since 1962, has for years been mired in a severe economic crisis marked by extended power cuts and shortages of fuel, medicine and food.

It has now also been cut off from critical oil supplies from Venezuela -- whose leader was toppled in a deadly US military strike last month -- and from Mexico under the threat of US tariffs.

The resulting shortages have threatened to plunge Cuba into complete darkness, with power plants struggling to keep the lights on.

No foreign fuel or oil tanker has arrived in Cuba in weeks, experts in maritime transport tracking have told AFP.

Emergency measures kicked in on the island this week to conserve its fast-dwindling fuel stocks. The government shuttered universities, reduced school hours and the work week, and slashed public transport as it limited fuel sales.

On Monday, Sheinbaum said Trump's "unfair" measures would "strangle" an already teetering economy.

Her country has been mulling how to send oil to Cuba without incurring punishing tariffs.

"We will continue supporting them (Cuba) and taking all necessary diplomatic actions to restore oil shipments," Sheinbaum said Monday.