Putin said Ukraine's presidential election campaign was being conducted in an unacceptable manner and Russia would not view its results as legitimate if the race continued in the same manner.
"The presidential race is being run in an absolutely unacceptable way...If everything continues in this way, then of course we cannot recognise as legitimate what is happening and what will happen after May 25."
UKRAINE - Three pro-Russian militia were killed and 13 injured in clashes with Ukrainian troops in the south-east city of Mariupol on Wednesday night, according to Ukraine's interior ministry.
Interior minister Arsen Avakov said on Thursday that the separatists were killed during an attempt to storm a Ukrainian base by some 300 people, armed with guns and petrol bombs. None of the interior ministry troops defending the base on the Sea of Azov were injured, Avakov said on his Facebook page.
"After attackers threw incendiary devices and molotov cocktails at the military unit and opened fire at guard posts; the national guards fired warning shots," Avakov said.
Only when the attack continued did the soldiers turn their weapons directly on the assailants, Avakov stated. Sixty-three people were detained and security officials confiscated weapons, communication devices and "Russian cellphones", the statement said.
The operation was still ongoing with additional police patrolling the port city and special forces being helicoptered in as reinforcements, Avakov said.
The latest attack follows the seizure of administrative buildings by pro-Russian separatists in Mariupol and other towns across the restive east of the country. The attack, the deadliest since separatist rebellions broke out in Russian-speaking eastern areas of Ukraine over 10 days ago, came hours before high-level talks to resolve the crisis were due to start on Thursday in Geneva, involving top diplomats from Russia, Ukraine, the United States and the European Union.
Ukraine is hoping to placate Russia and calm hostilities with its neighbour even as the US prepares a new round of sanctions to punish Moscow for what it regards as fomenting unrest.
"I think we still have a chance to de-escalate the situation using diplomatic means," Ukraine's foreign minister, Andrii Deshchytsia, told reporters late on Wednesday. "And we are trying hard."
But Deshchytsia said the diplomatic discussions must also be tempered with efforts "to look for a more concrete and adequate response to Russia's plans and actions".
US officials played down any expectations that the meetings in Geneva would yield a breakthrough or Russian concessions meaningful enough to avoid new US penalties.
With Ukraine struggling to contain a pro-Russian uprising in its eastern region bordering Russia, Barack Obama's administration is preparing additional sanctions against Moscow and a boost in aid for the Ukrainian military in the coming days, US officials said Wednesday.
The sanctions are likely to target more wealthy individuals close to the Russian president, Vladimir Putin, and the entities they run, while military aid could include medical supplies and clothing.
"Each time Russia takes these kinds of steps that are designed to destabilise Ukraine and violate their sovereignty, there are going to be consequences," Obama told CBS News on Wednesday night.
"Mr Putin's decisions aren't just bad for Ukraine. Over the long term, they're going to be bad for Russia."
US military aid was expected to stop short of body armour and other equipment for Ukraine's troops. The Obama administration is reluctant to send weapons and ammunition, as Kiev has requested, amid fears that lethal supplies would be seen as an escalatory step and trigger a more aggressive response from the estimated 40,000 Russian forces massed on its border with Ukraine.
Despite the diplomatic freeze between Moscow and Kiev, a senior state department official said Ukraine's negotiators planned to try to assuage Russia's concerns during Thursday's talks. Deshchytsia and his team were expected to brief Russia and the other diplomats on what Kiev was doing to transfer more power from the central government to the regions, including letting local areas keep more of their funding and elect their own leaders.
The Ukraine diplomats were prepared to field questions from negotiators and even seek Russia's advice on how to quell concerns in Moscow about the rights of Russian-speaking minorities in Ukraine and the approaching 25 May presidential elections to ensure they are inclusive for all candidates.
So far, the military movements and two initial rounds of sanctions against Russians and Ukrainians accused by the west of stirring up the unrest have done little to ease tensions.
Officials said a full-scale Russian invasion of eastern Ukraine would result in broad US and European sanctions on key Russian economic sectors, including its powerful energy industry. But European nations are divided on whether to limit its access to Russia's oil and gas supplies, and a vote to sanction must be unanimous among the EU's 28 member states.
The sanctions that could be levied in the aftermath of the Geneva meeting were expected to focus on Putin's close associates, including oligarchs who control much of Russia's wealth, as well as businesses and other entities they control. It was unclear whether those sanctions would change Putin's calculus, given that the US and the Europeans already have launched targeted sanctions on people in Putin's inner circle.