NATO has declared it is ready to respond to any threat following reports that Iran considered carrying out attacks against United States military targets in Europe.

"NATO is prepared to address any threat and will always do what is necessary to defend all Allies," a NATO official said in a statement on Wednesday.

The unnamed official also said NATO intercepted four ballistic missiles launched by Iran toward Turkiye this year, citing the incident as evidence that the alliance's "deterrence defence posture is strong and effective" against external threats.

The statement came in response to a report by a financial newspaper claiming Iran had considered striking US military locations across Europe.

The reported plans have raised concern in southeastern Europe, where several nations host military installations that support US operations.

Bulgaria's parliament approved the use of the Bezmer Air Base as a refuelling station for US military aircraft in July and authorised approximately 250 US military personnel to be stationed there until October.

Interior Minister Ivan Demerdzhiev told the country's national news agency that Bulgaria had taken "all possible measures" at the base in coordination with the Ministry of Defence. He said the steps were designed to address a threat level higher than that currently indicated by security services.

"We are constantly monitoring the situation and developments and will continue to take precautionary measures that are far more extensive and intensive than the level of threat indicated by the information available at the moment," he said.

Iranian Foreign Minister Abbas Araghchi spoke with his Bulgarian counterpart shortly after the US was granted permission to use the base. According to sources, he used the call to remind her that "any party that participates in any way in carrying out a military attack against Iran will certainly have to accept responsibility for its consequences."

A British airbase in Cyprus is the only European location struck by a drone since the start of the US-Israel war on Iran in February.

Iran has since launched retaliatory attacks on US bases and targets across the Middle East and has repeatedly warned that any nation assisting military actions against Tehran risks becoming a target.

The US and Iran signed a memorandum of understanding in June aimed at ending hostilities. The agreement set a deadline for the two sides to reach a permanent settlement, but that deadline expired on Monday. No negotiations are currently under way.