By Nailia Bagirova and Lucy Papachristou BAKU (Reuters) -Post-mortems on two Azerbaijani brothers who died in Russian police custody have shown that they were beaten to death, authorities in the South Caucasus country said on Tuesday as tensions rose sharply between Moscow and Baku.

The deaths of two brothers in Russian custody have laid bare a diplomatic rift with Azerbaijan, as it challenges Moscow's dominant role in the South Caucasus and establishes itself as a regional power.

Azerbaijan on Tuesday launched a criminal investigation into the deaths of two Azerbaijani nationals who were allegedly tortured and killed in