Iran has warned Bulgaria against allowing the US to use its territory for military operations against Iran.
The Bulgarian government has said it will seek parliamentary approval to station up to eight US tanker aircraft at an air base in the Balkan country, which is a member of NATO and the EU.
Iranian foreign ministry spokesperson Esmail Baqaei said on Tuesday any activity that facilitates US attacks on Iran would amount to complicity in what he described as "aggression and war crimes", according to Iranian state media.
Baqaei urged Bulgaria not to become "an accomplice of aggressors and lawbreakers", they said.
The tanker aircraft are mainly used for airborne refuelling of other aircraft.
Bulgaria's government aims to station the US aircraft at the Bezmer military facility, about 260 km (160 miles) southeast of the capital Sofia.
The US and Iran have intensified strikes since an interim ceasefire deal signed a month ago unravelled, raising the possibility of a return to all-out war.
