The EU and the US say they will impose “further costs” on Russia over its actions in Ukraine, as unrest continues in the east of the country.EU foreign ministers said on Monday they would expand a list of names targeted by EU sanctions.

Tension has been rising in Ukraine, with pro-Russian activists occupying buildings in more eastern towns.

Also on Monday, the US said a Russian fighter jet repeated close-range passes over a US warship in the Black Sea.

US President Barack Obama and his Russian counterpart Vladimir Putin spoke by phone on Monday evening.

Mr Putin told Mr Obama that allegations of Russian interference in Ukraine were “based on questionable information”.

The unrest in the east was the result “of the unwillingness and inability of the leadership in Kiev to take into account the interests of the Russian-speaking population”, the Kremlin said.

The White House said Mr Obama told Mr Putin of his “grave concern” about Russian support for armed separatists and he urged the Russian leader to use his influence with pro-Moscow groups to convince them to leave occupied buildings.

He warned: “The costs Russia already has incurred will increase if those actions persist.”

After a meeting in Luxembourg, EU foreign ministers said they had decided to expand “the list of those subject to assets freezes and visa bans”.

Earlier, UK Foreign Secretary William Hague had told reporters that he believed the seizure of official buildings in eastern Ukrainian towns and cities “is something that is being planned and brought about by Russia”.

Moscow denies fomenting the unrest.

EU foreign ministers also approved measures to provide up to 1bn euros (£800m; $1.4bn) of economic assistance to Ukraine.

The ministers backed “the temporary reduction or elimination of customs duties” for Ukrainian exports to the EU.

In Washington, the US signed a $1bn loan guarantee agreement for Ukraine after a meeting between US Treasury Secretary Jacob Lew and his Ukrainian counterpart, Oleksandr Shlapak.

UN appeal

Earlier on Monday, pro-Russian activists attacked another official building in eastern Ukraine, ignoring a deadline to leave or face eviction by Ukrainian forces.

A crowd stormed a police station in the town of Horlivka, near Donetsk, taking control of the building.

Interim President Olexander Turchynov had said Ukraine was preparing an “anti-terrorist operation” against gunmen occupying government buildings in a number of other towns and cities, but the deadline appeared to pass with no large-scale action from Kiev.

Mr Turchynov has condemned “aggression” from Russia and suggested the UN could assist in any “anti-terrorist” operation.

This would be highly unlikely as Russia has a veto on the Security Council, which would have to authorise any such action.

However, Mr Turchynov also said Kiev was “not against” a vote on the future of the country, a key demand from protesters.

The pro-Russian groups who have seized government buildings in eastern regions are demanding local referendums on either increased local rights or an option to join the Russian Federation.

The interim president suggested Kiev would be open to moving from a republic into a federation and giving broader rights to Ukraine’s Russian-speakers.

But he appeared to suggest a national vote as opposed to several regional ones. Many observers say the outcome of a national referendum would be uncertain because most people in Kiev and the Ukrainian-speaking west reject the idea of federalisation.

Putin’s ‘concern’

Russian Foreign Ministry Sergei Lavrov said on Monday it was not in Russia’s interests for Ukraine to break up but added that Moscow wanted all citizens of the country to be given equal treatment by Kiev.

He also denied allegations that Russian agents had been fomenting unrest in eastern Ukraine and said he was seeking an explanation from the US about a visit by CIA director John Brennan to Kiev.

The White House confirmed that Mr Brennan was in Kiev over the weekend but said the trip was routine.

Mr Putin’s spokesman told Interfax he had been “receiving many appeals from eastern Ukraine… calling on him to help in some form”, and that he was watching developments “with a great deal of concern”.

Ukraine’s foreign ministry said on Monday that it had proof Russian forces had been behind the “separatist operation” and said it would present its evidence at a meeting in Geneva later this week.