A senior Trump administration official said Monday that former Citgo executives detained for four months in Venezuela are being used as “hostages” and “bargaining chips” by President Nicolas Maduro’s government.

Venezuelan authorities detained six Citgo executives in November, after the U.S. imposed sanctions limiting the flow of corporate cash from the Houston-based company to Venezuela. At least five of the detained executives are naturalized U.S. citizens.

The executives -- Citgo’s former CEO and five vice presidents -- were arrested shortly before Thanksgiving and replaced by new executives led by CEO Asdrubal Chavez, a cousin of former Venezuelan President Hugo Chavez, and others believed to be allied with Maduro.

The ousted executives quickly faded from headlines last year as family members decided not to seek press coverage, but their case and the current situation at Citgo have been probed quietly by congressional staff.

After months of relative quiet, the group's detention was denounced Monday during a White House conference call held to discuss sanctions against a Venezuelan government cryptocurrency.

“The individuals that are being held in Venezuela, whether dual U.S.-Venezuelan citizens from the Citgo company or the U.S. citizen Joshua Holt, are being held from our perspective illegally, illegitimately by the government of Venezuela,” the senior official said.

Holt, a former Mormon missionary, was arrested in 2016 as a possible spy.

“We have clearly noted to the government of Venezuela that they must be released immediately," the official added.

The administration official did not offer specifics, but said the detained men are being used as bargaining chips, and referenced a visit to the U.S. this month by Rafael Lacava, the governor of Venezuela's Carabobo state. Lacava previously was reported to have sought talks with members of Congress to discuss Holt.

“These individuals have been illegitimately arrested and detained and have been used as bargaining chips," the official said about Holt and the Citgo executives. "As we saw with the Venezuelan state governor's visit to Washington several weeks ago, these individuals are bargaining chips that the Maduro dictatorship is using as hostages, and we are still calling for them to be released.”

The jailed former Citgo executives initially stood accused of negotiating a loan on poor terms. But defenders see their detention as pretext, and say the pending loan wasn’t a bad deal considering Citgo’s shaky outlook — the Russian government-owned Rosneft is looking to let go of its 49.9 percent stake in the company to avoid U.S. sanctions.

The purge of Citgo executives came shortly after the U.S. government imposed sanctions in August to prevent Citgo from being used for new loans on behalf of Venezuelan state oil company PDVSA, which owns a 50 percent stake in Citgo, and to restrict distribution of dividends and other profit-sharing. The firing and detention of Citgo executives was followed by a shakeup in Citgo’s legal department.

Citgo declined to comment on the White House characterization of former executives as being “hostages” in Venezuela, or discuss what role if any the company is taking to achieve their release.

“Citgo is not available for comment at this time,” said spokesman Ray Fohr.

A spokeswoman for Venezuela's embassy in Washington did not immediately respond to a request for comment.

Although the case received little press attention, a group of congressional staff met with the detainees’ families and current Citgo leaders this month. Staff working for Sens. Ted Cruz, R-Texas, Marco Rubio, R-Fla., John Cornyn, R-Texas, Bill Cassidy, R-La., and Rep. Pete Olson, R-Texas, have been involved with investigating the detention and pressing Citgo to do more to win their release, according to three sources.

After the senior administration official's remarks on Monday, Rubio’s office confirmed involvement with briefings at the State Department and regular conversations with the detainees’ families, in addition to discussions with current Citgo executives.

Other congressional offices declined to comment or did not respond to inquiries.

The Wall Street Journal reported in November that U.S. citizens detained in Venezuela include Citgo’s former vice president for refining Tomeu Vadell, former vice president for supply Jorge Toledo, former vice president for shared services Jose Luis Zambrano, and former Corpus Christi refinery director Alirio Zambrano. Former CEO Jose Angel Pereira and head of public affairs Gustavo Cardenas also were arrested.

Cardenas became a U.S. citizen in June after living in the U.S. for 14 years, a person close to Cardenas told the Washington Examiner.

This article was updated, in part to reflect that Gustavo Cardenas is a U.S. citizen.