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

President Claudia Sheinbaum of Mexico, formerly Cuba's second-largest oil supplier, has protested against the humanitarian impact of Trump's threats to impose tariffs on any country sending crude to Cuba.

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

They brought fresh and powdered milk, meat, cookies, beans, rice and personal hygiene items, according to the Mexican foreign ministry.

"Mexico has always been in solidarity with Cuba," Cuban Marila Garcia, 52, told AFP at Havana's Malecon waterfront, from where the ships were visible.

For his part, fisherman Eliecer Rodriguez, 34, thanked Mexico for being "the only country that is responding right now" to Cuba's urgent needs.

Hundreds of tons more powdered milk and beans were awaiting shipment from Mexico, according to authorities there, as Chile and Russia also promised Thursday to send help.

- 'Ready to fall' -

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, 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 this week to conserve the island's fast-dwindling fuel stocks. The government shuttered universities, reduced school hours and the work week, and slashed public transport as it limited fuel sales.

Staffing at hospitals was also cut back.

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 and taking all necessary diplomatic actions to restore oil shipments," Sheinbaum vowed.

- 'Dramatic situation' -

Chile under outgoing leftist President Gabriel Boric said Thursday it would also be sending aid to Cuba.

"This is a humanitarian issue, beyond the political characteristics its regime may have. What matters to us is meeting, as far as possible, the needs of the Cuban people," said Foreign Minister Alberto van Klaveren.

He added the aid was monetary, "which really no one could object to," and would be channeled through multilateral organizations.

In Moscow, the pro-government Izvestia media outlet reported that Russia would supply Cuba with oil as part of what it termed a humanitarian effort.

Russia's ministry of economic development told Izvestia that "as far as we know, Russia is expected to soon supply oil and petroleum products to Cuba as humanitarian aid."

On Monday, the Kremlin, a traditional ally of Cuba, accused the United States of attempting to "suffocate" the island nation.

In a statement from Geneva Thursday, a group of UN human rights experts slammed what they called an "extreme form of unilateral economic coercion" on the part of Washington.

"The US executive order imposing a fuel blockade on Cuba is a serious violation of international law and a grave threat to a democratic and equitable international order," the experts said in a statement.