By Matthias Williams and Pavel Polityuk
KIEV (Reuters) - Ukrainian investigators fear corruption probes could get buried because the national anti-corruption bureau will soon be flooded by thousands of old cases and recently passed legislation could further hobble their work.
Their comments spotlight Kiev's patchy record on fighting corruption, which has delayed billions in aid from international donors who have supported Ukraine since the 2014 Maidan protests brought pro-Western forces to power.
They come after the NABU anti-corruption bureau launched an investigation this week into an allied crime-fighting agency over extortion allegations.
From Monday, 3,500 cases that were registered before December 2015 will be transferred from the prosecutor's office to NABU, which include for example investigations that may pertain to former Donald Trump campaign chief Paul Manafort.
NABU began life in late 2015 and was given an exemption on investigating cases that opened before its creation, which expires on Monday. NABU wants the exemption extended, saying its 200-strong team of detectives cannot cope with the extra work.
In an interview with Reuters on Thursday, NABU spokeswoman Svitlana Olifira said there was a risk that "all current investigations by (NABU) detectives may be blocked".
Serhii Horbatiuk, head of special investigations at the general prosecutor's office, said the old cases would be sent to NABU to ensure no-one looks at them. His investigations include two cases related to Manafort's work in Ukraine.
"They will simply lie around and not be looked at," he said in an interview at his office. "My opinion is that this is done deliberately to ensure that crimes linked to former senior officials are either simply not investigated, or obstacles are created that prevent it (the investigation)."
"The restructuring (of law enforcement) is being used to ensure investigations don't take place," he added.
Neither NABU nor Horbatiuk accused anyone by name of trying to block investigations.
NABU appealed to President Petro Poroshenko to veto legislation passed in October which it believes will also harm investigations.
The law, according to NABU, will put too strict limits on the time allowed for an investigation before it can be dismissed, while also making it more cumbersome for police to obtain permission from courts to open probes.
"We urge the president to examine this bill thoroughly and to refrain from signing the current version," Olifira said, saying the bill could "bring about the collapse of Ukraine's whole law enforcement system."
The president's office did not immediately respond to a request for comment.
(Additional reporting by Margaryta Chornokondratenko and Sergei Karazy; Editing by Richard Balmforth)