US Vice-President Joe Biden is to meet Ukraine’s new leaders in a show of support for the pro-Western government.
Mr Biden is due to discuss the upcoming elections with the country’s interim prime minister and president during his visit to Kiev.
A phone conversation between the US secretary of state and Russia’s foreign minister earlier led to both sides blaming the other over the crisis.
Meanwhile, funerals are due to take place for three men shot on Sunday.
The men were killed during a raid on a checkpoint manned by pro-Russian separatists near the town of Sloviansk in eastern Ukraine.
The circumstances remain unclear. The local separatists said the attack was carried out by ultra-nationalist Right Sector militants. Kiev called it a “provocation” staged by Russian special forces.
Russian Foreign Minister Sergei Lavrov accused the Kiev authorities on Monday of breaking last week’s Geneva accord on resolving the Ukraine crisis.
He said the Kiev government – not recognised by Moscow – had not moved to disarm illegal groups, especially the Right Sector.
The authorities in Kiev say they were surprised by Mr Lavrov’s remarks and blame Russia for the instability.
In a phone call, Mr Lavrov urged US Secretary of State John Kerry “to influence Kiev, to prevent hotheads there from provoking a bloody conflict,” according to the Kremlin.
Meanwhile, the US state department said Mr Kerry “urged Russia to take concrete steps to help implement the Geneva agreement, including publicly calling on separatists to vacate illegal buildings and checkpoints”.
Judging by the contrasting accounts of its contents, the conversation simply led to both sides blaming the other for the fact that very little has changed in Ukraine since agreement was reached last Thursday, says the BBC’s David Willis in Washington.
The US has drawn up plans for further economic sanctions should Russia fail to make good on its Geneva commitments, our correspondent adds.
Appeals for unity
The 17 April Geneva accord was agreed at talks between Russia, Ukraine, the EU and US. It demanded an immediate end to violence in eastern Ukraine and called on illegal armed groups to surrender their weapons and leave official buildings.
Pro-Russian militants are still holding official buildings in at least nine towns and cities in the Donetsk region of eastern Ukraine.
The interim authorities in Kiev said they had suspended operations against pro-Russian militants over Easter, and appealed for national unity.
They promised to meet some of the demands of pro-Russian protesters, which include the decentralisation of power and guarantees for the status of the Russian language.
The US State Department released a series of photos of soldiers in eastern Ukraine on Monday, which it says show that some of the fighters are Russian special forces.
The US vice-president will meet Ukraine’s interim PM Arseniy Yatsenyuk and acting President Olexander Turchynov on Tuesday.
Mr Biden is expected to announce technical support to the Kiev government, including economic and energy-related assistance.
The White House said President Barack Obama agreed Mr Biden should make the two-day visit to Ukraine’s capital to send a high-level signal of support for reform efforts being pushed the new pro-Western government.
The elections on 25 May are seen as a crucial step in leading Ukraine out of the country’s deepest political crisis since its independence in 1991.
Ukraine has been in turmoil since last November, when Kiev was gripped by protests against President Viktor Yanukovych over his rejection of an economic pact with the EU. He was toppled in February and fled to Russia.
Russia then annexed Crimea following a regional referendum that approved joining the Russian federation. The annexation provoked international outrage.
Russian President Vladimir Putin on Monday submitted a bill to parliament to establish a gambling zone in Crimea.
The president approved a law making it easier for people in former Soviet republics to apply for Russian citizenship.
He also signed a decree to rehabilitate Crimea’s Muslim Tatars and other ethnic minorities who suffered during the rule of Soviet dictator Josef Stalin.
The 300,000-strong Tatar community – which makes up 15% of Crimea’s population – opposed the peninsula’s incorporation into Russia last month.