US Defence Secretary Lloyd Austin is on a two-day vist to Bulgaria for talks with Bulgarian PM Kiril Petkov and Defence Minister Dragomir Zakov.
They will discuss US-Bulgarian defence cooperation and NATO's plans to boost its eastern flank amid Russia's invasion of Ukraine.
Petkov has ruled out providing military aid to Ukraine but said his country, a NATO ally, will continue to provide humanitarian assistance.
"Bulgaria will continue to do everything possible to aid the Ukrainians in this hard war and this huge aggression that the Russian Federation has been inflicting on them," he told reporters.
"But being so close to the conflict, right now, military aid for Ukraine won't be possible.”
Bulgaria, which does not border Ukraine but has received thousands of refugees, has agreed to host a new contingent of NATO troops as part of the alliance's push to reinforce its eastern flank.
A battlegroup of up to 1,000 troops under the operational command of NATO's Supreme Allied Commander Europe will be established.
The US Defence Secretary thanked Petkov for his support.
"We also talked about the things that our forces are doing together and certainly we remain grаteful for his leadership and to Bulgaria for hosting a battlegroup here," Austin said.
Meanwhile, protesters rallied in a park near Bulgaria's Defence Ministry while the talks were taking place.
Surrounded by police, they chanted and held up banners, while some waved Bulgarian and Russian flags.