Nato Secretary General Mark Rutte assured Bulgaria of the alliance's support on August 20 amid concerns over a possible Iranian threat to US military assets in the country.
Rutte told Prime Minister Rumen Radev in a phone call that Nato forces and capabilities in the region were ready to respond to any threat to member states, the Bulgarian government said, BTA reported.
"Any attack on a Nato member state will be treated as an attack on the Alliance," the government quoted Rutte as saying.
The call followed a Financial Times report that Iranian forces had assessed the possibility of striking US military assets in Southeast Europe, including Bulgaria, citing two Iranian sources.
Bulgaria last month approved the use of Bezmer air base for US refuelling aircraft. Authorities have tightened security around the base but said there was no direct threat to national security.
The United States currently has two tanker aircraft at Bezmer, while Bulgaria has authorised up to eight aircraft and 250 personnel.
Greece said it was ready to consider a Bulgarian request for air-defence assistance, including the possible deployment of a Patriot missile battery.
Greece deployed a Patriot battery and F-16 fighter jets to support Bulgaria's air defence earlier this year, but later withdrew them.