Biden told Ukrainian political leaders that the US stands with them. "You face some very daunting problems and some might say humiliating threats are taking place."
Biden added: "The opportunity to generate a united Ukraine and getting it right is within your grasp. And we want to be your partner and friend in the project. We're ready to assist."
He said the US also stands ready to help Ukraine become energy-independent instead of relying on Russian supplies. "Imagine where you'd stand today if you could tell Russia to 'keep your own gas'."
Biden met Ukrainian President Oleksandr Turchynov and MPs from all regions of Ukraine. He was expected to announce a new package of assistance for Ukraine's faltering economy, which has suffered from the abrupt withdrawal of Russian support.
Biden's arrival coincided with a further escalation of the confrontation between the new Government and pro-Russian groups in the east.
US President Barack Obama has threatened more sanctions on Moscow if the Geneva accord is not implemented soon, beyond those already imposed by the US and the European Union. White House press secretary Jay Carney said yesterday that Washington was ready to make good on its threat, warning that "if progress is not made in coming days we will impose further costs".
On Monday, gunmen who control the town of Slaviansk in the Donetsk region detained Irma Krat, a Ukrainian journalist. She was blindfolded and placed before a camera for a video that was posted online yesterday.
"I was arrested for allegedly opposing the referendum," she said in the video, referring to the vote that separatists plan to hold on the status of the Donetsk region by May 11. "But this is speculation. I came to Slaviansk to tell the impartial truth."
Watch: Agreement to calm Ukraine tensions
The video was posted by Life News, a Russian website with close links to the security services. A man in camouflage gear whose name was given as Pavel said the journalist was being investigated for "war crimes" in the Kiev protests.
Krat, 29, appeared composed yesterday when she briefly met journalists at the occupied security service building in Slaviansk. She said she was detained while reporting on the shootings in Slaviansk in which three pro-Russian protesters were killed.
Slaviansk was in mourning yesterday after the killing of three separatists. Pavel Pavlenko, Alexander Siganov and Sergei Rudenko died in what appears to have been an attack on the checkpoint they were manning.
Russia's Foreign Ministry and Vyachelsav Ponomarev, the new "people's mayor" of the town, have blamed the attack on Pravy Sektor - a Ukrainian nationalist street militia that was prominent in the revolution.
Sergei Lavrov, the Russian Foreign Minister, accused Ukraine's Government of failing to curb the militia, describing the "crime" in Slaviansk as proof Kiev had broken the Geneva agreement. "All the signs show that Kiev can't - or maybe doesn't want to - control the extremists who continue to call the shots," he said. "Steps are being taken, above all by those who seized power in Kiev, which crudely violate the accords reached in Geneva."
However, Western governments point out that Russia's allies in eastern Ukraine still occupy government buildings in nine towns and cities across Donetsk, in breach of the Geneva agreement. Ponomarev, who heads the pro-Russian movement in Slaviansk, said his men were holding eight prisoners, including Krat, on suspicion of spying for Pravy Sektor or other hostile groups. He issued a stern warning to all foreign journalists, saying they must report what he considered the truth.
- additional reporting AFP, AP
