A helicopter participating in an initiative by the European Union’s border-monitoring agency Frontex to curb illegal immigration in the Aegean yesterday received a series of emergency signals via Turkish radar, warning that the aircraft had entered Turkish air space, Greek defense officials said. The signals were sent just before 7 a.m. as the aircraft had been flying over the islet of Farmakonisi. The Lithuanian pilots ignored the signals and continued with their patrol duties, the defense officials said. The Foreign Ministry in Athens responded by lodging a complaint with Ankara. It is the sixth time this year that Turkey has sent emergency signals by radar to Greek or foreign aircraft flying in the Aegean. A significant spike in violations of Greek air space and harassment of Greek fighter jets by Turkish aircraft in recent months has strained ties between Athens and Ankara.