A U.S. Navy fast-attack submarine arrived in Cuba's Guantanamo Bay on Thursday, hard on the heels of a Russian flotilla's arrival in Havana 24 hours earlier.

The USS Helena, one of around two dozen Los Angeles-class nuclear-powered, conventionally armed boats, was making "a routine port visit...while conducting its global maritime security and national defense mission," the U.S. Southern Command (SOUTHCOM) said in a statement on social media.

A group of Russian Navy vessels arrived in Cuba on Wednesday, a show of force by Moscow amid increasing tensions between the U.S. and Russia.

The vessels – the Russian frigate Admiral Gorshkov, nuclear-powered submarine Kazan, a rescue tug, and an oil tanker – entered Havana harbor with ceremonial firing from the Gorshkov and an 18th century colonial fort. The vessels are on a five-day official visit to Cuba in a show of ties between Moscow and Havana, according to CNN.

Three Russian ships and a nuclear-powered submarine are expected to arrive in Cuba this week ahead of military exercises in the Caribbean, officials said. While the exercises aren't considered a threat to the U.S., American ships have been deployed to shadow the Russians, U.S. officials told CBS News.

The US Treasury's regulatory amendment to the so-called Cuban Assets Control Regulations, announced on May 28, has caused quite a stir in Washington and beyond. For the first time since the revolution in the 1950s, Cuban entrepreneurs can open a bank account in the United States and access it from Cuba.

The changes are intended "to promote internet freedom in Cuba, support independent Cuban entrepreneurs in the private sector, and expand access to certain financial services for the Cuban population," the Treasury Department said in a statement.

Alejandro Fonseca stood in line for several hours outside a bank in Havana hoping to withdraw Cuban pesos from an ATM, but when it was almost his turn, the cash ran out. He angrily hopped on his electric tricycle and traveled several kilometers to another branch where he finally managed to withdraw some money after wasting the entire morning.

“It shouldn’t be so difficult to get the money you earn by working,” the 23-year-old told The Associated Press in a recent interview.

Two of the most progressive Democrats in the House of Representatives led a small delegation on an undisclosed trip to Cuba last week to discuss "human rights," despite the worsening border crisis affecting everyday Americans here at home, according to a new report late Tuesday.