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.

Officials say police in Azerbaijan's capital of Baku searched the offices of Russia’s state-funded news outlet Sputnik amid rising tensions between the two countries after the deaths of two ethnic Azerbaijanis during a police raid last week in a Russian city.