KYIV (Reuters) - Ukraine's top anti-corruption investigator is under pressure to find the source of a suspected leak from his staff that compromised an important investigation, an episode that is testing a pillar of Kyiv's Western-backed reform effort.

The National Anti-Corruption Bureau of Ukraine (NABU) has stepped up a wartime campaign against graft to prove the country's credentials for joining the European Union, with which it is expected to formally start accession talks next week.

The alleged leak, acknowledged by NABU last month, has rekindled fears of political interference in the independent force, set up with Western help in 2015 as a prized vanguard against high-level corruption.

It has also jolted an agency already under intense scrutiny from a vigilant and often impatient civil society as public tolerance of corruption has dropped during Russia's 28-month-old invasion.

Director Semen Kryvonos declined on Thursday to attend a parliamentary committee hearing on his response to the incident after a media report this month suggested his chief deputy was a possible origin.

The deputy, who was suspended last month at his own request, declined a Reuters request to comment through a NABU spokesperson.

Kryvonos, who was appointed in March 2023, has promised a transparent investigation and said in a statement that commenting on an ongoing probe was "premature" and distracted from investigative efforts.

NABU has not publicly detailed the alleged leak, but a law enforcement source said it led to a loss of evidence in a high-profile case involving a government road-building project after a suspect was alerted to its developments.

The anti-graft agency, along with a special prosecutor, has earned praise in recent months for targeting a former Supreme Court chief, government ministers and an ex-deputy head of President Volodymyr Zelenskiy's administration.

Some experts, however, have said the alleged leak reflects the difficulty of weeding out old habits that took root as a result of traditionally weak state institutions.

ANTI-CORRUPTION DRIVE WATCHED CLOSELY That was reinforced this month when anti-corruption authorities caught a lawyer at a widely respected Kyiv law firm allegedly offering a $200,000 bribe to transfer away a case against his client.

Establishing a solid track record of fighting graft will be central to Kyiv's hopes of keeping its EU accession hopes alive. International donors are also looking for signs that large sums of aid will be safe.

Ukraine ranked 103rd out of 180 countries in Transparency International's latest Corruption Perceptions Index.

Andrii Borovyk, executive director of the organisation's Ukraine office, said the investigation into the suspected leak was an opportunity for NABU to burnish its transparency credentials by cleaning its own house if violations are found.

"The question is not only about this leak," he said. "The question is ... whether there were other leaks of information, when they were, and why they were happening."

Borovyk added that communicating more clearly with the public would also be crucial.

A Western diplomat, speaking on condition of anonymity, said NABU could be trusted to finish its investigation and that heaping too much pressure on Ukraine's overworked anti-graft agencies could backfire.

"It only benefits those who do not want these institutions to do their work," the diplomat said.

(Editing by Timothy Heritage)

By Dan Peleschuk