At least three people were killed in a gunfight in the early hours of Sunday, April 20 near a Ukrainian city controlled by pro-Russian activists, shaking an already fragile international accord that was designed to avert a wider conflict, according to Reuters.

The incident triggered a war of words between Moscow and Ukraine's Western-backed government, with each questioning the other's compliance with the agreement, brokered last week in Geneva, to end a crisis that has made Russia's ties with the West more fraught than at any time since the Cold War.

The activists said armed men from Ukraine's Right Sector nationalist group had attacked them. The Right Sector denied any role, saying Russian special forces were behind the clash.

Failure of the Geneva agreement could bring more bloodshed in eastern Ukraine, but may also prompt the United States to impose tougher sanctions on the Kremlin - with far-reaching consequences for many economies and importers of Russian energy.

U.S. Vice President Joe Biden will meet with Ukraine's acting president, Oleksander Turchinov, and Prime Minister Arseny Yatseniuk in Kiev on Tuesday during a two-day stay in Ukraine.

Biden will consult on developments in eastern Ukraine and discuss international efforts to strengthen Ukraine's economy and assist Kiev "in moving forward on constitutional reform, decentralization, anti-corruption efforts, and free and fair presidential elections on May 25th," the White House said on Sunday.

Under the deal signed in Geneva last week, the European Union, Russia, Ukraine and the United States agreed that illegal armed groups would go home in a process to be overseen by Europe's OSCE security watchdog.

So far, the pro-Russian activists have shown little sign of budging from public buildings in the east, although there was some hope of progress after Kiev said it would not move against the separatists over Easter, and international mediators headed to eastern Ukraine to try to persuade them to disarm.

But the shootings near Slaviansk - already a flashpoint for tensions between Ukraine's rival camps - are likely to make that task even harder, hardening the view of the many Russian speakers in eastern Ukraine that they cannot trust Kiev, Reuters said.