CARACAS, Venezuela (AP) — The government of President Nicolas Maduro on Thursday nullified the newly named board of Venezuela's prized Citgo refineries in the U.S. amid a political battle for control of the country, saying opposition leaders had no right to appoint them.

The battle over the nation's most valuable foreign asset comes as Maduro fends off opposition leader Juan Guaidó, whose claim to Venezuela's presidency is backed by the United States and more than 50 other nations.

The opposition-run National Assembly appointed the 15-member Citgo board early this year after Guaidó declared presidential powers, arguing that Maduro's re-election was illegitimate. Maduro maintains power with backing from the military and allies such as Cuba, Russia and China.

Citgo is valued at an estimated $8 billion and includes three refineries in Louisiana, Texas and Illinois, in addition to a network of pipelines. Citgo is also at the center of court battles, such a lawsuit filed by Crystallex, which seeks to liquidate Citgo for payment following a disputed takeover of the Canadian mining firm by Venezuela's late President Hugo Chavez.

Russ Dallen, head of the Miami-based Caracas Capital Markets brokerage firm, said U.S. courts have already weighed in on who controls Citgo, deferring to the U.S. government's recognition of Guaidó as Venezuea's president, therefore recognizing his board appointments.

"It does show they are stepping up the attacks on the opposition," Dallen said of the Maduro government's latest move. "They're going to try to make it as painful as possible for anybody who sides with Guaidó."

State comptroller Elvis Amoroso, who made the announcement on state TV, also said the ad-hoc board members are banned from leaving the country and their Venezuelan bank accounts have been frozen. But it is unclear whether any of them continue to live in the South American country.

Amoroso also said another five leading figures opposed to Maduro have been banned from politics for 15 years. They including former Attorney General Luisa Ortega, former Caracas Mayor Antonio Ledezma and three opposition lawmakers, all of whom are living in self-imposed exile.

They were accused of "concealment and falsification of information in the presentation of affidavits," Amoroso said. So far this year, Maduro's government has stripped 18 opposition lawmakers of their immunity, exposing them to criminal prosecution in a sigh of heightened political tensions.