Washington DC - President Donald Trump unleashed a four-letter broadside at Venezuelan leader Nicolas Maduro on Friday, as he said the leftist strongman had offered major concessions to ease tensions with Washington.

"He has offered everything, you're right. You know why? Because he doesn't want to fuck around with the United States," Trump said when a journalist at the White House asked about reports that Caracas had floated de-escalation plans.

Washington accuses Maduro of heading a drug cartel and has deployed significant military assets – including stealth warplanes and seven US Navy ships – as part of what it says are counter-narcotics efforts in the region.

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Earlier this week in a show of force, US-based B-52 bombers circled over the Caribbean off the coast of Venezuela for several hours, data from tracking website Flightradar24 showed.

The US military said Friday that the flights demonstrated "the US commitment to proactively deter adversary threats, enhance crew training, and ensure the global force readiness."

The buildup has sparked fears in Caracas that Washington's ultimate goal is regime change in Venezuela.

The South American country is the alleged origin of some of the at least six purported drug-smuggling vessels struck by US forces as part of an unprecedented campaign that has killed more than two dozen people.

"We attacked a submarine, and that was a drug-carrying submarine built specifically for the transportation of massive amounts of drugs," Trump told journalists Friday in response to a question about reports of a strike that left several people alive.

US Secretary of State Marco Rubio, sitting alongside Trump, would not confirm if there were survivors, saying more information would likely be released later in the day.

Washington has not provided evidence to support its assertion that the targets of its strikes are drug smugglers, and experts say the summary killings are illegal, even if they target confirmed narcotics traffickers.