The US added to its sanctions on the Venezuelan government on Tuesday, designating 15 aircraft as blocked property of the country's state oil firm.

In addition to transport, two of the aircraft were involved in unsafe and unprofessional incidents with US military aircraft in 2019, the Treasury Department said.

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The US unveiled its latest sanctions against the government of Venezuelan President Nicolas Maduro on Tuesday, announcing that the Treasury Department's Office of Foreign Assets Control designated 15 aircraft as blocked property of the state oil company, Petroleos de Venezuela, or PDVSA.

Several of the aircraft were used to "transport senior members of the former Maduro regime in a continuation of the former Maduro regime's misappropriation of PdVSA assets," according to the release. The US no longer considers the Maduro government legitimate, instead recognizing Juan Guaidó, president of the country's national assembly, as interim president.

The only Venezuelan official named on Tuesday was Manuel Salvador Quevedo Fernandez, the oil minister, who was sanctioned by the US last year. He used a PDVSA aircraft to attend an OPEC meeting in the United Arab Emirates in late summer 2019, the release said.

A Bombardier Learjet 45 in which the US government says Venezuela's PDVSA has an interest. US Treasury

The aircraft designated on Tuesday were not only used for transportation, however. According to the release, "several of these aircraft have been operated in an unsafe and unprofessional manner in proximity to US military aircraft, while in international air space."

In winter 2019, a Venezuelan Bombardier Learjet 45 "flew in close proximity to a US military aircraft over the Caribbean Sea," the release said. In spring last year, a different Bombardier Learjet 45 "attempted to interfere with a US military aircraft in the northern Caribbean Sea" during what the release described as "a joint operation conducted by PdVSA and the Venezuelan Integrated Air Command."

It was not immediately clear when these encounters took place or what US aircraft were involved. The Treasury Department was not immediately able to provide additional information.

A spokesman for US Southern Command, which is responsible for military operations in and around Central and South America, said in an email that "for operational security reasons" they would not discuss the details of the specific events mentioned in the Treasury Department's release.

"However we acknowledge the seriousness of the incidents," the spokesman added, saying the command "values safety of flight" and expects all air crews to operate professionally and in a safe manner.

'Very unsafe, very unprofessional'

A Venezuelan Su-30 Flanker fighter jet seen from a US Navy EP-3 Aries II patrol aircraft over the Caribbean, July 19, 2019. US Navy/YouTube

Dangerous encounters between US and Venezuelan aircraft are not unheard of.

In July 2019, a US Navy EP-3 Aries II, an intelligence and reconnaissance aircraft, was "approached in an unprofessional manner" by a Venezuelan Su-30 Flanker fighter jet and "aggressively shadowed ... at an unsafe distance in international airspace for a prolonged period of time, endangering the safety of the crew and jeopardizing the EP-3 mission," Southern Command said in a statement at the time.

Southern Command said the EP-3 was following international standards and rules. The US Navy also released several videos of the encounter.

The US patrol plane "was flying in international airspace as part of our intelligence and monitoring mission, so we can better understand the context, so we can understand what to plan for and what to anticipate and how to work inside the US whole-of-government efforts" to respond to the situation in Venezuela, US Navy Adm. Craig Faller, head of Southern Command, said in October when asked about the July incident.

"Those Venezuelan pilots were very unsafe, very unprofessional, flew at a very close distance, and we were quite vocal about releasing the video that showed that happen. We keep an eye on this closely. If there were other instances of unsafe, unprofessional activity by the Venezuelans, we'd report that before they had a chance to spin it in a way that was not accurate."

"I think it's representative of their lack of professionalism in some of their forces, unfortunately, and as we go forward we will monitor that closely," Faller added.

The Flanker is a Russian-made fighter jet, 24 of which Venezuela ordered in 2006 and now operates alongside US-made F-16 fighters acquired earlier. Moscow has provided support for those jets and other weapons systems used by Venezuela's military, underscoring the international dynamics affecting the situation in Venezuela.

This story has been updated with comments from US Southern Command.