Russian Prime Minister Vladimir Putin on Thursday expressed regret that the leaders of the USSR did not fight to the last to prevent its collapse two decades ago.

"The USSR should have started timely economic reforms and changes as well as reforms to strengthen democratic change in the country," ex-KGB agent Putin told Russian television viewers in a phone-in.

"They should have consistently, fearlessly and steadfastly -- without burying their heads in the sand or waving their arses in the air -- fought for the territorial integrity of our country," he said.

Putin's comments were his latest expression of regret that the USSR collapsed 20 years ago in December 1991, after he in 2005 memorably described its demise as the "greatest geopolitical catastrophe" of the 20th century.