Ukraine's prime minister has told leaders in the east of the country that he is committed to allowing regions to have more powers.

Arseniy Yatsenyuk told reporters he favours a peaceful solution to the stand-off with Russia, but left it unclear how his ideas differ from the demands of protesters occupying government buildings in the east or from Moscow's advocacy of federalisation.

He also left the door open to storming the buildings occupied by armed men, even though a two-day deadline announced earlier this week has passed.

The officials who met Mr Yatsenyuk in Donetsk did not include representatives of the protesters. They asked Mr Yatsenyuk to allow referenda on autonomy for their regions, but not on secession.

"There are no separatists among us," said Gennady Kernes, mayor of Kharkiv, Ukraine's second-largest city, where protesters occupied a government building earlier in the week.

Ukraine's eastern industrial heartland was the support base for Kremlin-friendly president Viktor Yanukovych, who was ousted in February after months of protests. Last month, the Crimea region voted to secede and was annexed by Russia.

Moscow ratcheted up the pressure on Ukraine yesterday when president Vladimir Putin warned European leaders of a risk to the gas supplies going through Ukraine. He has threatened that Russia could shut off shipments to Ukraine if it fails to pay its mammoth debts.

Mr Putin's spokesman Dmitry Peskov told Russian news agencies today that Moscow has not heard from the countries to which Mr Putin sent a letter.

Protesters in the eastern cities of Donetsk and Luhansk are still occupying government buildings and calling for referenda on regional autonomy that could prefigure seeking annexation by Russia.

Mr Yatsenyuk said the grievances of eastern Ukraine would be appeased by the upcoming constitutional reform that will "satisfy people who want to see more powers given to regions". He mentioned abolishing Kiev-controlled local administration as one of the steps to decentralise the country.

The protesters in Donetsk, who have held the regional administration building since Sunday, initially called for a referendum on secession but later reduced the demand to one on autonomy, with the possibility of holding another later on whether the region would remain part of Ukraine or seek to become an autonomous region within Russia.

The eastern parts of Ukraine have a high proportion of Russian speakers and many of them fear that the acting government that took over when Mr Yanukovych fled will repress them.

Kiev and Western officials in turn claim that Moscow is whipping up tensions in the east, with the aim of establishing a pretext for sending in troops. Russian foreign minister Sergey Lavrov denied any participation by Russia in the events in the east, saying on Rossiya television: "Our servicemen aren't there. Our agents aren't there."

He said Russia does not aim to annex any parts of eastern Ukraine and "we want Ukraine to be whole with its current borders". He also dismissed earlier calls by the eastern Ukraine protesters for Russia to send in peacekeeping forces.

Russia is calling on Ukraine to change its constitution to become a federalised state in which regions would have more control of their own affairs. Ukraine's government has resisted federalisation, saying that would lay the groundwork for the country's break-up.

Nato secretary general Anders Fogh Rasmussen again urged Russia to pull back its troops from Ukraine's borders, and added that Nato is taking legitimate steps to deal with the instability created by Russia's "illegitimate" actions.

Speaking in Prague, Czech president Milos Zeman called on Nato and the European Union to take robust pre-emptive action to deter Russia from invading other parts of Ukraine after its takeover of Crimea. He stopped short of giving details.

In north-west Romania, 450 US and Romanian troops were conducting joint military exercises, flying US F-16 fighter jets alongside Romanian ones. Romania, Russia and Ukraine all border the Black Sea.

In a bid to apply pressure on Russia, the European Union has warned Moscow of further sanctions for ratcheting up tensions in Ukraine.

A senior European Union official said the 28-nation bloc's foreign ministers would consider broadening the list of people sanctioned to deter Russia from further destabilising the situation in eastern Ukraine.

The source said ministers at a meeting in Luxembourg on Monday are not expected to decide new sanctions but could agree on ordering the preparation of a new list of targets.