They didn't. And moreover, really had no way to co-operate in a practical sense. In fact, I'm not even sure why they were 'allies'.
They did. Up until the beginning of 1944 there were dwindling numbers of voyages by surface blockade runners and commencing 1943 voyages by U-boats and even before Italy withdrew from the war voyages by Italian submarines to Penang. After Penang became too dangerous voyages set out for Djakarta/Batavia. As late as November 1944 U-219 and U-195 arrived with radars, jet and rocket engines, blueprints etc. Among their cargo were twelve broken down V-2 (A-4) rockets.
In a 1982 TV interview former general Kawashima indicated that 2000 kilograms of Uranium oxide were successfully shipped to Japan.
In his memoirs of armaments Mnister Albert Speer a flight to Tokyo via the polar route with a Junkers Ju-390 was described.
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