The US has promised Ukraine non-lethal military aid but urged Kiev to act in a "measured and responsible way" in responding to unrest in the east.

The American offer came before Thursday's talks over the fate of eastern Ukraine and as low morale among the country's soldiers became evident in confrontations with pro-Moscow separatists.

Negotiations between Russia, Ukraine, the EU and US began in Geneva amid low expectations and battling narratives over what is going on the ground, where at least one separatists was killed during a reported attempt to storm a Ukrainian base in the south-eastern town of Mariupol.

Asked if he was expecting any progress, the US secretary of state, John Kerry, simply shrugged. He held separate meetings on Thursday morning at the Intercontinental Hotel with the EU foreign policy chief, Catherine Ashton, the Ukrainian foreign minister, Andrii Deshchytisa, and his Russian counterpart, Sergei Lavrov, before all four began a plenary session.

Deshchytisa said he had come with "optimism and goodwill" but Lavrov did not hold a separate meeting with him. The ballroom set aside for the closing press conference has been decked out with the US stars and stripes but no other flags. Asked if Lavrov would also be speaking there, a US diplomat said: "All I know is we hired this hall. If he wants to come, he can hire it from the hotel after we're finished."

Speaking in a televised call-in, the Russian president, Vladimir Putinm, said: "I think the start of today's talks is very important, as it's very important now to think together about how to overcome this situation and offer a real dialogue to the people."

The White House press secretary, Jay Carney, noted that the Geneva meeting was the first time for the four foreign ministers, known as the "contact group", have met since the crisis erupted in February.

"We expect the conversation to focus on the need to de-escalate, the need for Russia to demobilise its troop presence on the border with Ukraine, the need for armed separatist groups within Ukraine to disband and disarm, and for the Ukrainian government to discuss the measures it intends to take when it comes to constitutional reform and decentralisation," Carney said.

He said the US was considering requests from Kiev for support for the army, but stressed it would not be lethal aid in the form of arms or ammunition. Press reports have suggested it would include items like uniforms, but stop short of body armour.

The offer came after an Ukrainian effort to reassert control in the eastern town of Kramatorsk on Wednesday ended in fiasco. Confronted by angry local residents, the soldiers looked uncertain of what to do. They ended up handing their armoured cars to the separatists, who paraded them as trophies under Russian flags in the nearby city of Slavyansk.

Briefing journalists on Air Force One, Carney urged restraint on Kiev, saying: "It is certainly appropriate for Ukraine to take action to restore law and order, but we believe that they should continue to do so in a measured and responsible way."

In his remarks Putin denied there were any Russian forces in eastern Ukraine.

"It's all nonsense, there are no special units, special forces or instructors there," the Russian leader said. But he did admit that Russian soldiers had been in Crimea before it was annexed by Moscow. At the time he had insisted the soldiers in unmarked uniforms who swept across the peninsula had been self-defence groups who may have got their green fatigues from local stores.

On Thursday he said Russian soldiers had been in Crimea to protect the local population and provide security for referendum, which produced an overwhelming vote for secession and joining Russia.

Carney did not allege that Russian troops were in eastern Ukraine but insisted the separatists were receiving Russian support.

He said: "These groups have clearly co-ordinated their actions and received support from the outside. And in many cases they don't have the support in any visible way of the populations in the cities or regions where they are occupying buildings."

The US and EU are encouraging Deshchytsia to emphasise Kiev's readiness to provide reassurances and increased autonomy to the eastern provinces where there are large Russian-speaking populations. But Kiev refuses to accept the sort of federalism being promoted by Moscow, which it sees as a cover for partition and the break-up of the country.