Iran’s foreign ministry spokesman warned Bulgaria on Tuesday against allowing the U.S. to use its territory for military operations against Iran.

The Bulgarian government has said it will seek parliamentary approval to station up to eight U.S. tanker aircraft at an air base in the Balkan country, which is a member of NATO and the European Union.

Ministry spokesman Esmail Baghaei said any activity that facilitates U.S. attacks on Iran would amount to complicity in what he described as “aggression and war crimes”, according to Iranian state media.

Baghaei urged Bulgaria not to become “an accomplice of aggressors and lawbreakers”, they said.

The tanker aircraft are mainly used for airborne refueling of other aircraft.

Bulgaria’s government aims to station the U.S. aircraft at the Bezmer military facility, about 160 miles southeast of the capital Sofia.

The U.S. and Iran have intensified strikes since an interim ceasefire deal signed a month ago unraveled, raising the possibility of a return to all-out war.