Iran's foreign ministry spokesman warned Bulgaria on Tuesday 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 European Union.
Ministry spokesman Esmail Baghaei said 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.
Baghaei urged Bulgaria not to become "an accomplice of aggressors and lawbreakers," they said.
The tanker aircraft are primarily used to refuel 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 unraveled, raising the possibility of a return to all-out war.
Iran warns Azerbaijan, Turkey against aiding US
Iranian officials have held talks with both Turkey and Azerbaijan officials, warning them against participating in attacks on Iran, Azerbaijani media site News.AZ reported earlier on Monday, citing Iran's senior military advisor Yahya Rahim Safavi.
"We explicitly stated that Turkey or Azerbaijan are not our targets," Safavi stated. "However, there must be no attacks launched against us from their territory; otherwise, we will target them."
The talks took place after an unspecified drone-related incident, which Safavi described as a "clear provocation," adding that he had ordered an internal investigation.
News.AZ also cited Safavi saying that Turkey had told Iran that the Incirlik Air Base would not be used in attacks on Iran.
Incirlik is located near Adana and hosts the US Air Force's 39th Air Base Wing as well as other NATO partner aircraft.
Azerbaijan denies role in US, Israeli attacks on Iran
Azerbaijan also told Iranian officials that there were no US or Israeli military bases within its territory, Safavi said.
In June, Iranian hardline outlet Raja News alleged that some attacks on Tehran and Karaj came from the direction of the Caspian Sea and claimed that Azerbaijan may have been used as a route for Israeli drones, small aircraft, or fighter jets entering Iranian airspace.
Azerbaijan has denied allowing its territory or airspace to be used against Iran, according to the same Raja News report, which said Azerbaijani Foreign Minister Jeyhun Bayramov spoke with Iranian Foreign Minister Abbas Araghchi after the attacks.
Jerusalem Post Staff contributed to this report.