Cuba’s former leader Fidel Castro offered another show of support to embattled Venezuelan President Nicolás Maduro this week, writing him an open letter that was published in Cuba’s state newspaper Granma. Castro assured Maduro that Venezuela would “never accept threats” from abroad, in reference to escalating tensions between Caracas and Washington.

The letter, dated Monday and published Tuesday, came ahead of a meeting of the Bolivarian Alliance of the Americas (ALBA), a leftist bloc founded by former Venezuelan President Hugo Chávez. The summit would discuss “the unusual policy of the U.S. government against Venezuela and ALBA,” Castro wrote, referring to sanctions U.S. President Barack Obama placed on seven Venezuelan officials last week.

The executive order Obama signed included language that referred to Venezuela as a threat to national security, which has put Maduro and his allies on the defensive despite White House officials saying it was standard language for sanctions programs.

“The Bolivarian Republic of Venezuela has stated precisely that it is always ready for a peaceful and civilized discussion with the U.S. government, but it will never accept threats or impositions on the country,” Castro’s letter continued. He also praised Venezuela’s military, which began exercises over the weekend as a show of defense against the perceived threat from the U.S.

“Today Venezuela has the best equipped soldiers and officials in Latin America,” Castro wrote. “When you met with officials in recent days, one could appreciate that they were ready to give their last drop of blood for their homeland.”

The letter comes a week after Granma published much briefer remarks from Castro to Maduro, following the U.S. announcement of new sanctions. “I congratulate you for your brilliant and brave speech against the brutal plans of the U.S. government,” he wrote. “Your words will go down in history as proof of what humanity can and should know about the truth.” Cuban President Raul Castro had previously noted his “unconditional support” for Venezuela in light of the sanctions.

Cuba has been Venezuela's closest ally in the region for years: Their governments' ideological alignment has been bolstered by Venezuela's steady supply of oil to the island. But Cuba’s support for Venezuela comes at a tricky time when Washington and Havana are trying to hammer out the details of their own rapprochement. U.S. and Cuban negotiators ended a third round of talks in Havana Tuesday to draw up plans for re-establishing diplomatic relations.

The U.S.-Cuba thaw has been widely heralded by Latin American governments, including that of Maduro. But Cuba's loyalty to Venezuela has added another complicated layer to the talks, and Washington’s increasingly rocky relationship with Caracas threatens to eclipse discussions at next month’s Summit of the Americas, which takes place in Panama.