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.

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 ethnic Azerbaijanis amid allegations of torture during a police raid last week in the Russian city of Yekaterinburg has quickly escalated into a diplomatic confrontation highlighting difficulties in Moscow's relations with its former Soviet republics.