Vladimir Putin's invasion of Ukraine has met with tougher resistance and higher costs than the Kremlin imagined but the Russian leader may still be undeterred in seeking to realise his ambitions.

One month into the invasion launched on February 24, Moscow has faced heavy military losses and unprecedented sanctions in exchange for relatively meagre progress on the ground.

But analysts say a variety of factors, including the changing seasons and even an upcoming new Russian draft conscription intake, could encourage Putin to press the operation for months to come.

His ultimate aim, according to many Western analysts and security sources who have spoken to AFP, remains control of the entire country up to the Polish border.

But the stalled progress military progress on the ground could encourage him to pursue a negotiated settlement, with Russia seeking minor concessions from Kyiv that Moscow could then claim as a victory.

One initial Kremlin aim that appears to have been shelved for now is the immediate overthrow of President Volodymyr Zelensky, with calls for regime change now less prominent in Kremlin rhetoric.

"The initial plan, which was probably a lightning war to take control of Kyiv very quickly and bring down the Ukrainian government, did not work," said Marie Dumoulin, director of the Wider Europe programme at the European Council on Foreign Relations (ECFR).

"The most likely outcome now is that Putin will intensify his efforts and switch to a dirty war strategy to increase the human cost for the Ukrainians and force them to capitulate," she added.

The tenacious Ukrainian resistance and the heavy losses inflicted on Russia -- which for weeks has not disclosed a military death toll -- mean that this war is very different to the operation to annex Crimea in 2014 and the intervention from 2015 in Syria to bolster the regime there.

Negotiation channels are open meanwhile, with Zelensky saying he is ready to talk to Putin, the opposing delegations holding several rounds of talks in Belarus, and Turkey standing by to step in as a mediator.

Russia could yet take steps such as the use of chemical weapons or targeting humanitarian convoys, analysts say.

Moscow has faced heavy military losses and unprecedented sanctions in exchange for relatively meagre progress on the ground
Moscow has faced heavy military losses and unprecedented sanctions in exchange for relatively meagre progress on the ground AFP / BULENT KILIC

Another critical factor will be if troops from Russia's ally Belarus enter the war in a bid to swing the balance on the northern front.

"Putin still thinks this will not go on like this and he will end up imposing his will by sheer mass whatever the resistance," said Frederic Charillon, professor of international relations at Clermont Auvergne university.

Faced with the difficulties of the Russian army on the ground and the cascade of sanctions against Russia, Putin is "however moving more and more towards a war of destruction and punishment", he added.

"The question is not so much what he wants to get, but how and at what price," said Tatiana Stanovaya, founder of the R.Politik analysis firm and a scholar at the Carnegie Moscow Center.

"It will take time, will cause more problems, but he is convinced that he has no choice and that he is on a historic mission" of restoring Russian influence.

The minimum territorial demands for Russia -- along with Ukraine renouncing NATO membership -- are likely to be Ukrainian recognition of annexed Crimea as part of Russia and of the breakaway Donetsk and Lugansk regions as independent.

Russia could also be eager to take control of territory in southeast Ukraine -- and notably the besieged city of Mariupol -- to link Crimea to Russia by land.

Its troops could then press on westwards to the Black Sea port of Odessa.

Zelensky has said that the status of Crimea and the pro-Moscow breakaway statelets could be discussed, but with the proviso that any major changes would need to be put a referendum, which would likely doom any such idea.

While the increasing Russian losses could put Putin under domestic pressure, so far there has been no sign of any critical split within the Russian elite.

"The key for Putin is power, pressure and victory," said Abbas Gallyamov, a former Kremlin speechwriter and now a political consultant. "He can't pull back without having something to show for it.

"He needs an agreement on the neutrality of Ukraine. But this is obviously not enough. He also wants recognition of Crimea and of the republics of pro-Russian separatists from Lugansk and Donetsk."