US vice-president Joe Biden was to begin a two-day visit to Ukraine on Monday amid Russian "outrage" over a deadly weekend shootout in the rebel east that shattered a fragile Easter truce.
Washington has warned Moscow that time is running out for the implementation of an accord signed along with Ukraine and the European Union in Geneva last Thursday that was meant to ease tensions in the crisis-hit country.
Moscow in turn has warned that it will not tolerate further US sanctions if the deal falls apart, while stressing that it has tens of thousands of troops massed on Ukraine's doorstep.
Biden was expected to reassure Ukrainian leaders of America's continued support during his visit to Kiev.
The United States and its NATO allies have bolstered military deployments in eastern Europe. Washington and the Brussels have also pledged billions to shore up Ukraine's battered economy.
In Ukraine's restive east, the situation appeared calm early Monday, with pro-Kremlin separatists still in control of public buildings they have occupied for over a week.
"There was no shooting overnight," Yevgen Gorbik, a rebel wearing camouflage and a military cap and standing at a barricade in the flashpoint town of Slavyansk, told AFP.
"We will only shoot if attacked," he added.
Gorbik summed up the bellicose posturing and political jockeying by saying: "Currently, we have a virtual president in Ukraine, a virtual army, and a virtual war."
Watch video: Deadly shootout in east Ukraine shatters Easter truce
Real bullets in gunbattle
On Sunday, though, the bullets were real in a shootout at a roadblock near the rebel-held town of Slavyansk that killed at least two of the separatist militants.
Pro-Moscow insurgents in Slavyansk and the Kremlin blamed the attack on Pravy Sektor ("Right Sector"), an ultra-nationalist group that was at the vanguard of Kiev street protests that forced the February ouster of pro-Moscow former president Viktor Yanukovych.
But Ukrainian officials and Pravy Sektor dismissed the allegation as Russian "propaganda". They ridiculed the purported discovery of a business card belonging to the leader of Pravy Sektor in one of the attackers' cars, which Russian media had claimed was proof of the group's involvement.
The self-styled leader of Slavyansk, Vyacheslav Ponomaryov, said a total of three rebels and two attackers had died in the attack, though AFP saw the bodies of only two militants.
Ponomaryov announced a midnight-to-dawn curfew in the town and appealed to Russian President Vladimir Putin to deploy troops to the region as "peacekeepers", or at least send weapons to help fight the "fascists".
Russia's foreign ministry issued a statement expressing its "outrage" at the violence. It called for Kiev to comply with the Geneva accord, which demands all "illegal armed groups" surrender their weapons and leave occupied public buildings.
OSCE to triple monitors
A spokesman in Kiev for the Organisation for Security and Cooperation in Europe, which is tasked with monitoring the implementation of the Geneva agreement, told AFP that there was so far "no confirmation" of the separatists leaving buildings they had seized.
Spokesman Michael Bociurkiw said the OSCE currently had 100 monitors in Ukraine, more than half of them in the east, but planned to triple that number this week.
The teams, he said, were encountering some difficulties in travelling around the separatist east, particularly into Slavyansk, because of insecurity and insurgent-manned roadblocks.
The White House said in a statement that Biden would "consult on the latest developments in east Ukraine" during his trip to Kiev.
On Monday he was due to speak with US embassy officials in the Ukrainian capital. On Tuesday he was to meet with the country's interim president, Oleksandr Turchynov, Prime Minister Arseniy Yatsenyuk and lawmakers.
The United States sees Russia as pulling the strings in the Ukrainian insurgency, notably by sending in Russian special forces to stir unrest and coordinate the rebels' actions.
President Barack Obama last week said Russia has "days" to prevail upon the rebels to abide by the Geneva accord, otherwise it risked more Western sanctions on top of those already levelled at Putin's inner circle.
Russia, which last month annexed Crimea after sending in Russian troops, retorted that Washington should not treat it like a "shameful schoolboy". It rejects any justification for further sanctions.
Putin himself, though belatedly acknowledging the Russian military played a role in Crimea, has denied his army is operating in east Ukraine.
Nevertheless, he asserts he has a "right" to send in forces to his eastern neighbour, which shares historical and linguistic ties with Russia.
A Kremlin spokesman shrugged off as "absurd" claims that Washington could sanction Putin directly, after an article in Britain's Times newspaper cited anonymous sources saying the United States could target Swiss bank accounts belonging to the leader that allegedly hold some $40 billion (29 billion euros).
Russia's ambassador to the United States, Sergei Kislyak, told Fox News on Sunday that sanctions represented a return to "Cold War mentality" but said Moscow could "withstand pressures".