Washington threatens further action against Russia if Vladimir Putin uses poll to legitimise military occupation, says Kiev envoy

America's ambassador in Kiev said the US would refuse to recognise next Sunday's "so-called referendum" in Crimea, and said Washington would take further steps against Russia if it used the poll to legitimise its occupation.

Geoffrey Pyatt said Barack Obama and the US secretary of state, John Kerry, had spent the weekend talking to European leaders. Obama also spoke to Russia's president, Vladimir Putin. The ambassador said the US and EU were in complete agreement that stronger sanctions could follow after next weekend's referendum, adding: "There is no daylight between us."

The ambassador said the White House was unbending in its view that Crimea was part of Ukraine. He said that in the runup to Sunday's referendum "gangs of pro-Russian thugs" were roaming the peninsula, beating up activists and creating an atmosphere of fear and intimidation. Without mentioning Moscow by name, Pyatt said there was also an "active campaign right now" to stir up dissension and division across the country.

Ukraine's interim prime minister, Arseniy Yatsenyuk, is due to travel to Washington on Wednesday for talks with Obama. The trip would be an opportunity to reaffirm the US's strongest support for the "new democratic Ukraine'", its integrity and the Ukrainian people, Pyatt said. They would also discuss Russia's invasion of Crimea.

His comments came after David Cameron and Angela Merkel agreed that any Russian attempt to legitimise Sunday's referendum in Crimea would result in further consequences, implying stronger sanctions.

The UK prime minister and the German chancellor agreed a statement after a working dinner in Hanover on Sunday night.

On Sunday, during separate telephone calls with Obama and Merkel, the Chinese president, Xi Jinping, urged a political solution to the crisis in Ukraine and for all parties to exercise calm and restraint.

"The situation in Ukraine is extremely complex, and what is most urgent is for all sides to remain calm and exercise restraint to avoid an escalation in tensions," China's foreign ministry on Monday cited Xi as telling Obama.

"Political and diplomatic routes must be used to resolve the crisis," Xi added. China had an "open attitude" towards any suggestions or proposals that could ameliorate the situation and was willing to remain in touch with all parties including the US, he said.

Xi told Merkel the Ukraine situation was "highly sensitive" and needed to be weighed carefully, according to a separate Chinese statement.