WASHINGTON (Reuters) - The United States on Thursday condemned the “cruel and indefensible” detention of executives from U.S. refiner Citgo in Caracas, said the top U.S. envoy for Venezuela, who also warned Russia over its support for Venezuelan President Nicolas Maduro.

FILE PHOTO: United States diplomat Elliott Abrams takes notes during a meeting of the U.N. Security Council called to vote on a U.S. draft resolution calling for free and fair presidential elections in Venezuela at U.N. headquarters in New York, U.S., February 28, 2019. REUTERS/Lucas Jackson

Venezuelan police seized the six executives who had been under house arrest, their families and an attorney said. Citgo is owned by Venezuelan state oil company PDVSA.

The executives “had been taken from house arrest by the regime’s intelligence agencies and we believe they are now detained at Helicoide prison” in Caracas, Elliott Abrams, U.S. Special Representative for Venezuela, told reporters in a briefing in Washington.

Abrams called the detentions “cruel and indefensible” and said their timing on Wednesday was “suspicious” as Venezuelan opposition leader Juan Guaido, recognized as the country’s rightful president by the United States and dozens of other countries, met with President Donald Trump at the White House.

Abrams said the U.S. government had made efforts to help get the executives released, without giving details.

He also warned Moscow about its support for Maduro, hinting at potential sanctions against Russia, a day after the Trump administration called on energy companies with ties to Maduro’s government to “tread cautiously.”

“As several administration officials have noted, the Russians may soon find that their continued support of Maduro will no longer be cost-free,” Abrams said, adding that others who profit from supporting Maduro should also heed the warning.

Russia’s Rosneft, India’s Reliance, Spain’s Repsol and U.S.-based Chevron have emerged as PDVSA’s main business partners since last year when the United States imposed the steepest sanctions yet on the Venezuelan state-owned company.

But following the Jan. 28, 2019 ban on U.S. companies importing Venezuelan crude, the United States has not followed through on threats to extend the sanctions to any foreign company doing business with PDVSA. Venezuela’s oil exports have risen in recent months, throwing Maduro a lifeline.

Rosneft, which became the largest intermediary for Venezuelan oil last year at time when few were willing to trade it, has shown no signs of backing off.

The United States to date has not imposed sanctions on Russia or Rosneft over their support for Maduro.

Abrams on Thursday declined to discuss Washington’s conversations with Rosneft, but promised more sanctions overall, without elaborating. “We see that sanctions have had and are having significant impact and sanctions will increase,” he said.