Pope Leo said on Sunday he was deeply concerned about rising tensions between the United States and Cuba, and he called for "sincere and effective dialogue" to prevent violence and further suffering for the Cuban people.

US President Donald Trump said last week tariffs would be imposed on imports from countries that supply Cuba with oil, ratcheting up the pressure on Washington's long-time foe after ousting Venezuelan President Nicolas Maduro, a key Cuban ally, in early January.

Trump said the tariff threat was necessary to protect "US national security and foreign policy from the Cuban regime's malign actions and policies."

Pope Leo said he had received reports "with great concern" of rising tensions between Cuba and the United States. He joined Cuban bishops in "urging those responsible to promote sincere and effective dialogue to avoid violence and further suffering for the Cuban people," in comments after his weekly Angelus prayer.

Last week Trump predicted that "Cuba will be failing pretty soon," adding that Venezuela, once the island's top oil supplier, had not recently sent oil or money to Cuba.

Cuba's Foreign Minister Bruno Rodriguez declared an "international emergency" in response to the US tariff warning, which he said constituted "an unusual and extraordinary threat."

On Saturday, Trump reiterated his call for Cuba to negotiate with the United States. "It doesn't have to be a humanitarian crisis," he told reporters aboard Air Force One en route to Florida.