Biden, who was the Obama administration's point person on Ukraine, called Kurt Volker, the special envoy for Ukraine appointed by President Donald Trump, a "solid, solid guy," RFE/RL said.
Read alsoVOA: U.S. special envoy for Ukraine confirms Russia talksBut Biden said Volker hasn't been given enough authority to be tougher on Ukraine's leadership, not only on corruption but also implementing the Minsk accords, the framework deal reached with Russia, Ukraine, France, and Germany to end the conflict.
Volker is "a solid, solid guy. But Kurt, to the best of my knowledge, does not have the authority, or the ability, to go in and say, 'If you don't straighten this up, you're out of here,'" Biden said.
Biden made the comments January 23 in an appearance at the Council on Foreign Relations in Washington.
The United States has been a stalwart supporter of Ukraine since 2013 when a series of street protests over closer integration with Europe evolved into a major confrontation with President Viktor Yanukovych, culminating in his ouster in February 2014.
The Obama administration imposed economic sanctions for Moscow's annexation of Ukraine's Crimean Peninsula and for its support of separatists in the war with Ukrainian forces that has killed more than 10,000 people in eastern Ukraine since April 2014.
But Washington has also struggled to push Yanukovych's successor, Petro Poroshenko, to institute major governmental reforms and clean up the country's endemic corruption.
Biden said he and other administration officials had to work hard to persuade European leaders to go along with U.S. sanctions on Russia.
"If, in fact, you do not show progress in terms of corruption, we are not going to be able to hold the rest of Europe on these sanctions," Biden said he told Ukrainian leaders. "And Russia is not going to roll across the line and take over the rest of the country with tanks. What they're going to do is take your economy over, and you're going to be absolutely buried. It's going to be done. And that's when it's all going to all go to hell."
"There is no pressure that I'm aware of… on the present leadership in Ukraine to hold them together" on implementing the Minsk accords, he said. "We were moving in that direction, but it now looks like the pressure is off and it requires day-to-day-to-day" reinforcement.
Neither Volker nor his advisers responded immediately to an e-mail seeking comment from RFE/RL.
The Trump administration policy toward Ukraine had been in doubt early on in his presidency, amid Trump's repeated calls for a more conciliatory approach toward Russia. During the 2016 election campaign, the Republican Party platform was reportedly changed to water down U.S. support for Kyiv.
However, with Congress and key national security officials showing strong backing for Ukraine, the Trump White House has largely continued his predecessor's policies.
Last year, the White House signaled it was moving forward on a long-delayed plan to supply Ukraine with more advanced weaponry, to bolster its forces fighting Russian-backed separatists.
