WASHINGTON (Reuters) - The United States on Tuesday blacklisted 15 aircraft belonging to Venezuelan state oil company Petróleos de Venezuela SA, or PDVSA, the latest in a series of actions targeting the socialist government of Venezuelan President Nicolas Maduro.

Washington accused the PDVSA aircraft of operating in an “unsafe and unprofessional manner in proximity to U.S. military aircraft, while in international air space,” the U.S. Treasury Department said in a statement.

“Several of these aircraft have been involved in the harassment of U.S. military flights in Caribbean airspace,” U.S. Secretary of State Mike Pompeo said in a statement.

Treasury said PDVSA aircraft have also been used to transport senior members of the Maduro government, including transporting blacklisted Venezuelan oil minister Manuel Quevedo to attend an OPEC meeting in the United Arab Emirates.

The United States slapped sanctions on the state oil company last year in an effort to oust Maduro. The latest action blocks Americans from making transactions that involve the aircraft.

Pompeo on Monday warned that the United States would take more action to support Venezuelan opposition leader Juan Guaido after meeting with him on the sidelines of a regional conference in Bogota.

Last January, Washington recognized Guaido as the OPEC nation’s legitimate interim president and began ratcheting up sanctions and diplomatic pressure on the Maduro government. A year later, Maduro remains in power, backed by the military as well as Russia, China and Cuba.