Mr Putin said appropriate conclusions were being drawn from the errors.

He was speaking as Russian forces carried out their heaviest land and air offensive for several weeks.

Mr Putin told Russian television that time and patience would be required to win the war.

He did not say how long it would take, but correspondents say his remarks appeared to be preparing the public for an extended campaign.

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Earlier in the week, a Russian general said the war would be over within two months, but Mr Putin was more circumspect.

"Delays will be determined by what can be justified militarily. We will act firmly but not with cruelty," Mr Putin said.

Hearts and minds

The Russian leader stressed that civilians would not be sacrificed for military gains, because local support was critical in the struggle.

"It is only by broadening our base of support among civilians who back the federal authority that we can obtain an outright success - not with carpet bombing," he said.

The Russian army's treatment of Chechnya's civilian population had earlier been criticised by a senior Russian human rights official, as reports emerged of heavy civilian casualties in Russia's bombardment of Grozny.

Human rights commissioner Oleg Mironov told Mr Putin that aid had to be sent urgently to civilians affected by the crisis.

The rebels have also been criticised by human rights activists, who said they had provoked Russian counter-attacks in civilian areas.

Human Rights Watch said Chechen fighters had also frequently beaten and threatened civilians trying to save their villages.

In a separate development, the Russian special envoy to Chechnya, Nikolai Koshman, said he was planning to move his administrative base into the republic within three days, to the second-largest city, Gudermes.

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