As Russia's invasion of Ukraine continues to threaten European security, the U.S. Defense Department approved the sale of eight F-16 fighter planes to Bulgaria on Monday in a $1.67 billion deal to bolster the country's air force.

"The proposed sale will improve Bulgaria's capability to meet current and future threats by enabling the Bulgarian Air Force to deploy modern fighter aircraft routinely in the Black Sea region," the Defense Security Cooperation Agency said.

The sale approval came amid continuing speculation that one of Washington's NATO allies could provide Ukraine with MIG-29 aircraft to fight Russian forces, and that such a move could involve the United States providing that ally with U.S. aircraft in exchange.

Bulgaria is one of the three NATO members that operate MIG-29s, which Ukraine pilots are capable of flying. The others are Poland and Slovakia.

But Pentagon spokesperson John Kirby, asked about the F-16 sale Monday, downplayed the suggestion it could be tied to Bulgaria providing MIG-29s to Ukraine.

During a visit to Bulgaria by U.S. Defense Secretary Lloyd Austin on March 19, Prime Minister Kiril Petkov said there was no such deal.

"But being so close to the conflict, I have to say that currently we will not be able to send military assistance to Ukraine," he said, adding that assistance would have to be approved by the Bulgarian parliament.