LISBON, Oct 9 (Reuters) - Portuguese Prime Minister Luis Montenegro argued on Friday the US use of the Lajes Air Base during the Iran conflict was fully legal and urged the judiciary not to make politicaljudgments, after prosecutors opened an inquiry last week into the government's authorisation to use the base.

The probe, triggered by a complaint from a leftist opposition party, adds a legal dimension to a politically sensitive issue. Several European nations have restricted or refused US military aircraft access to their airspace or bases in connection with the war in Iran.

The left-wing opposition has criticised the centre-right minority government for allowing US military transit flights to use the Lajes base, in the Atlantic Azores archipelago, during the Iran war. Spain, France, Italy, Austria and Switzerland have all imposed restrictions, much to Washington's chagrin.

"Obviously, everyone can make politicaljudgments about decisions, with the exception of the judiciary, which should not make political judgments," Montenegro told lawmakers during a parliamentary debate, a day after meeting US Secretary of State Marco Rubio in Lisbon.

He added that he respected the ongoing investigation and was "very calm as regards compliance with all applicable legal rules".

The government has said Portugal was not involved in the conflict and that the use of the Lajes base complied with international law, having been authorised only after assurances from Washington that any operation would be defensive, necessary and proportionate, and limited to military targets.

Portugal's prosecutors are bound by the principle of legality, requiring them to investigate alleged wrongdoing when there are sufficient grounds to do so, unlike in some jurisdictions where prosecutors have broader discretion to decide whether a case should proceed.

(Reporting by Sergio Goncalves; editing by Andrei Khalip and Alex Richardson)