The US vice president Joe Biden has pledged US support - and funds - for Ukraine as anti-Kiev militia in the country's east edged a step closer towards secession.

A "people's assembly" in Luhansk, where heavily armed militia have been occupying the security service headquarters for more than two weeks announced on Tuesday morning that they would hold a two-stage referendum on the region's future.

The armed men remain in the regional government buildings they have occupied for several weeks despite a peace deal struck in Geneva that called on pro-Russian rebels to turn in their weapons and vacate the buildings they had seized in nine cities.

Russian and western leaders have accused each other of not fulfilling their end of the bargain following a shootout in eastern Ukraine that left at least three dead this weekend.

Speaking in Kiev following meetings with the western-allied leadership, Biden told Russia on Tuesday that it was "time to stop talking and start acting".

He met members of the parliament, the acting president, Oleksandr Turchynov, and the acting prime minister, Arseniy Yatsenyuk, and warned that the country must "fight the cancer of corruption that is endemic in your system" before pledging $50m (£30m) to help Ukraine's government to carry out political and economic reforms, including $11m to help conduct the presidential election on 25 May.

Biden also announced an additional $8m in non-lethal military assistance for the Ukrainian armed forces, including bomb-disposal equipment, communications gear and vehicles.

In Luhansk, voting for a Ukrainian president will be preceded by a vote for independence. The first phase of the referendum, planned for 11 May, when voters will be asked whether the region should become an autonomous entity. A second phase, planned for 18 May, will ask whether Luhansk should be independent or join Russia.

Pro-Russian protesters in Donetsk have also promised to hold a referendum on the region's "sovereignty" by 11 May.

Ukraine's interior ministry has reportedly created a special "Timur" battalion to fight separatism in the Luhansk region. Although the unit would appear to take its name from its commander, army veteran and champion power lifter Timur Yuldashev, he said on Ukrainian television it was so named because Timur in the Uzbek language means "ironclad".

"I'll try to do everything so that this battalion will indeed become ironclad, so that it becomes a unit that can help fend off the threat of separatism in our Luhansk region and the breakup of our nation," Yuldashev said.

So far, police in Luhansk have allowed the building occupation and rallies outside to proceed unimpeded. The anti-terrorist operation announced by Kiev last week to regain control over the east of the country has achieved little success, with some soldiers defecting to the rebel side and surrendering six infantry fighting vehicles to militia in Slavyansk.

During his meeting with Biden, heavyweight boxer and Kiev mayoral candidate Vitaly Klitschko called on the US and Europe to adopt fresh sanctions against Russia that would "include all sectors of the economy and actually be painful". The US and its European partners, which have greater trade ties with Russia, have limited their sanctions to visa bans and asset freezes on Russian officials, an approach that hasn not noticeably affected the Kremlin's policy on Ukraine.

Klitschko also called on the US to help equip the Ukrainian military, which has reportedly suffered from shortages in manpower and supplies.

The US has sent the USS Donald Cook to the Black Sea for war games. A US navy spokeswoman said there was "no truth" to Russian media reports that US and Russian military dolphins could meet in open waters, following Russia's takeover of Ukranian combat dolphin unit when it annexed Crimea.Also on Tuesday, Russian authorities barred Crimean Tatar leader Mustafa Dzhemilev from all Russian territory for five years, citing a law banning foreign nationals who are accused of threatening public order. The ban on Dzhemilev, who left the peninsula for a trip to Kiev on Tuesday morning, came a day after the Russian president, Vladimir Putin, rehabilitated Crimean Tatars of political crimes they were accused of by Joseph Stalin, who deported most of the ethnic group during the second world war.