Relations between the United States and Cuba may have warmed since the two countries announced plans to restore full diplomatic ties in December, but Havana did not hold back on Tuesday when it called new U.S. sanctions against Venezuela, its closest ally, "arbitrary and aggressive," Agence France-Presse reported.

Cuban President Raúl Castro joined other leftist leaders in South America in closing ranks with his Venezuelan counterpart, Nicolás Maduro, whose government the Obama administration says has created "an extraordinary threat to the national security" of the United States.

Washington this week imposed sanctions against a number of Venezuelan officials linked to human-rights abuses, leading Cuba to renew its commitment to its Socialist ally; state-run media on the island published an official statement sharply condemning the U.S. decision.

"Cuba again reiterates its unconditional support and that of our people for the Bolivarian Revolution, the legitimate government of President Nicolás Maduro, and the heroic brotherly people of Venezuela," the statement said.

Revolutionary leader Fidel Castro, for his part, addressed Maduro in a brief message similarly published in state-controlled news outlets, the Washington Post noted. The elder Castro said he was impressed by remarks in which the Venezuelan president had blasted the sanctions on Monday night.

"Dear Nicolás Maduro," the 88-year-old wrote. "I congratulate you for your brilliant and brave speech in the face of the brutal plans by the United States government. Your words will go down in history as proof that humanity can and will know the truth."

The fact that the most visible displays of support for Maduro came from Fidel Castro and not from his brother Raúl -- who had worked out the diplomatic reconciliation with President Barack Obama -- was "telling," the Washington Post editorialized. "While Raúl Castro also regularly pledges to support the Venezuelan president, he does not have the same close personal relationship," the newspaper added.

Ecuador's foreign minister, meanwhile, warned on Monday that the Southern American UNASUR bloc would not allow foreign intervention or a coup in Venezuela, AFP noted. Maduro on many occasions has accused the United States of plotting his overthrow, and his mentor and predecessor, Hugo Chávez, was convinced that Washington was behind a failed 2002 coup against him.