US Secretary of State John Kerry has rejected an offer of talks with Russian President Vladimir Putin until Moscow engages with US proposals to tackle the crisis in Ukraine, BBC reports.
Mr Kerry told his Russian counterpart Sergei Lavrov that Moscow's military intervention in Crimea had made any negotiations extremely difficult.
US officials say there will be little to talk about if the referendum on Crimea's future goes ahead, amid Ukraine and the West say that the vote is illegal.
In a televised briefing with President Putin on Monday, Mr Lavrov said proposals made by Mr Kerry for a negotiated solution to the crisis are "not suitable" because they take "the situation created by the coup as a starting point", referring to the overthrow of Ukraine's pro-Russian President, Viktor Yanukovych.
Russian forces have strengthened their control over Crimea.