'It seems to be open season on anyone trying to highlight the appalling human rights abuses in Chechnya. It's high time the Russian government acted to stop these killings and prosecute those responsible,' the head of the New-York based international human rights organisation, Kenneth Roth, said.

Estemirova had earlier worked with journalist Anna Politkovskaya, who was shot dead in Moscow in October 2006, and human rights lawyer Stanislav Markelov, who was killed in the Russian capital in January this year.

Chechen President Ramzan Kadyrov said: 'A search for the criminals will be carried out not only during an official investigation, but also unofficially, according to Chechen traditions.'

Russian human rights activists will gather in Moscow's Novopushkinsky Square to pay tribute to Estemirova.