The United States said Thursday that cash-strapped Venezuela is paying Iran in gold to rebuild its troubled oil industry as it denounced what it saw as growing cooperation between its two adversaries.

Elliott Abrams, the envoy who leads US efforts to topple Venezuela's leftist leader Nicolas Maduro, said that Iran has been sending "more and more planes" to the South American nation, including this week.

"Our guess is that they are being paid in gold," he said at the Hudson Institute, a conservative Washington think tank.

"Those planes that are coming in from Iran that are bringing things for the oil industry are returning with the payments for those things: gold."

President Donald Trump's administration has slapped unilateral US sanctions aimed at ending oil exports from both Iran and Venezuela, major crude producers.

Venezuela has the world's largest proven oil reserves but analysts say that the sector operates well below capacity due to corruption and a lack of investment in upkeep.

Venezuela's economy has been collapsing, with millions fleeing as they lack basic goods, and Iran has also taken a hit from US sanctions after Trump pulled out of a denuclearization accord.

Abrams charged that Iran's role showed soft support for Maduro from Russia and China, which have stood by him despite Western pressure.

"One of the reasons I mention that is not just to show that Iran is playing an increasing role, but notice that it's cash," Abrams said.

"We know that Maduro has over the last year wanted Russian and Chinese additional loans, additional investments, and he has not gotten a dime," he said.

Secretary of State Mike Pompeo on Wednesday also highlighted the two US nemeses' cooperation, saying that "multiple" aircraft had been spotted in Venezuela from Iran's Mahan Air.

The carrier is under US and other sanctions for allegedly transporting fighters and weapons on behalf of Iran's elite Revolutionary Guards.

"These flights must stop and countries should do their part to deny overflights just as many have already denied landing rights to this sanctioned airline," Pompeo told reporters.

The relationship between Iran and Venezuela is not secret, with the Venezuelan foreign ministry saying that Maduro and Iranian President Hassan Rouhani agreed to step up cooperation in an April 13 phone call.

Maduro has withstood more than a year of US-led efforts to remove him and retains the support of the military.

Some 60 nations recognize opposition leader Juan Guaido as interim president due to wide reports of irregularities in Maduro's 2018 re-election.