The United States and European partners are stepping up efforts to draft tougher sanctions against Russia if it seeks to disrupt Ukraine's upcoming presidential elections, a US official said this afternoon.

Ukrainian officials are also working to hold another round of unity talks in eastern Ukraine on Monday, the senior State Department official revealed.

During talks in London today, US Secretary of State John Kerry briefed counterparts from Britain, France, Germany and Italy on the latest US efforts to draw up sanctions.

The West has pledged to support the 25 May presidential elections in Ukraine.

The US and its allies are working "to send a unified message to pro-Russian separatists and Moscow that any disruption of these elections will result in the next round of costs for Russia including sectoral sanctions," a US official told reporters.

There was "broad unity that if the elections are disrupted and Moscow's hand is behind that, that we need to move to sectoral sanctions".

"There was no dissent on that subject," the official stressed, asking not to be named.

US President Barack Obama has already drafted an executive order to impose sanctions on Russia across key sectors such as banking, energy, defence and mining.

The aim was "to use a scalpel rather than a hammer" and to focus on new investment in these sectors, the official said, adding "that there are a lot of things we can do to create bleeding".

The Russian economy "does have considerable vulnerabilities," the official said, adding European countries, which have greater economic and energy dependence on Russia than the US, were "becoming increasingly confident that there is a way to do this".

Ukrainian authorities are also planning to hold new unity talks in eastern Ukraine on Monday, although it has not yet been determined where.

The government in Kiev yesterday hosted the first round of so-called national unity talks under an OSCE initiative to try to resolve the deepening crisis on Europe's eastern flank and allow the 25 May vote to go ahead.

But crucially, the pro-Moscow rebels fighting against Kiev's rule in the industrial east of the country were not at the negotiating table, despite Western calls for the talks to be inclusive.

The State Department official said it was understood that on Monday "any political figure, any NGO figure who wants to participate can... the only requirement is that they renounce violence".

And she praised the Kiev talks as "a real forum for Ukrainians from all the over the country to express themselves politically".