NATO fighter jets intercepted a Russian spy plane over the Baltic Sea after it breached Estonian airspace, the alliance said Wednesday in the latest of such incidents amid tensions with Moscow over Ukraine.

The Ilyushin IL-20 "intelligence collection aircraft", which took off from the Russian Baltic coast enclave of Kaliningrad on Tuesday, was first intercepted by Danish F-16 jets when it approached Denmark, NATO said in a statement.

It then flew north towards non-NATO member Sweden which also sent jets to intercept the plane.

Nearly four hours later the plane flew towards NATO member Estonia and was detected in Estonian airspace "for a period of less than one minute, which represented an incursion of about 600 metres (yards) into NATO airspace," it said.

Portuguese F-16s spotted the Russian propeller plane and escorted it out of NATO airspace, the alliance said.

The Ukraine crisis has produced the worst east-west tension since the end of the Cold War.

NATO has stepped up its Baltic air patrols due to what it says is a surge in intercepts of Russian warplanes near Polish and Baltic airspace since Moscow's annexation of Crimea from Ukraine in March.

The Swedish Navy meanwhile has been searching for a suspected Russian submarine off the coast of Stockholm for nearly a week, although it said Wednesday it was pulling back some of its ships.