KIEV (Reuters) - A Ukrainian investigation of gas company Burisma is focused solely on activity that took place before Hunter Biden, son of former U.S. Vice President Joe Biden, was hired to sit on its board, Ukraine’s anti-corruption investigation agency said.

Ukraine's President Volodymyr Zelenskiy listens during a bilateral meeting with U.S. President Donald Trump on the sidelines of the 74th session of the United Nations General Assembly (UNGA) in New York City, New York, U.S., September 25, 2019. REUTERS/Jonathan Ernst

Separately, a senior official at the General Prosecutor’s office said that neither of the Bidens had been called for questioning in relation to this investigation.

Ukraine would open an investigation into the period when Hunter Biden was involved with Burisma if there were compelling new testimony in Ukraine, Nazar Kholodnytsky, the head of anti-corruption investigations at Ukraine’s Prosecutor’s Office, said on Novoye Vremya radio.

But it could not do so on its own initiative, based solely on comments currently being made in the United States, he said.

U.S. President Donald Trump asked Ukraine’s President Volodymyr Zelenskiy in July to probe whether Joe Biden tried to block an investigation into his son’s relationship with the company, which was drilling for gas in Ukraine.

Trump’s intervention is the subject of an impeachment investigation by the U.S. House of Representatives, with Democrats saying the Republican president was trying to push a foreign leader to smear Joe Biden, who is seeking to run against Trump at presidential elections in 2020.

The National Anti-Corruption Bureau of Ukraine (NABU) said an investigation was ongoing into permits granted by officials at the Ministry of Ecology for the use of natural resources to a string of companies managed by Burisma.

But it said the period under investigation was 2010-2012, and noted that this was before the company hired Hunter Biden.

“Changes to the board of Burisma Limited, which are currently the object of international attention, took place only in May 2014, and therefore are not and never were the subject of (the anti-corruption bureau’s) investigation,” the bureau’s statement said.

Hunter Biden was a director on Burisma’s board from 2014-2018, according to documents filed by the company in Cyprus, where it is registered.

The investigation into Burisma covers a period when Ukraine was governed by a Kremlin ally, Viktor Yanukovich. Burisma hired Hunter Biden after Yanukovich was toppled in a popular revolt in 2014 and replaced by a pro-Western government.

At the time, many Ukrainian firms were seeking to distance themselves from their relationships with the previous, pro-Moscow authorities, and some invited Western public figures to sit on their boards.

The NABU’s investigation related to the 2010-2012 period is not particularly active, Kholodnytsky added.

“At the moment, this case is up in the air, so to speak. Up in the air means that there is no active investigative work ongoing. At the moment, detectives and prosecutors do not understand what they are supposed to be investigating,” Kholodnytsky said.

Trump wants Ukraine to investigate the Biden family, accusing Joe Biden of strongarming the Ukrainian authorities, including the then President Petro Poroshenko, to fire General Prosecutor Viktor Shokin in 2016 to protect his son.

Shokin says he was fired to prevent him from investigating Hunter Biden, which the Biden family strongly denies.

Kholodnytsky said that were Poroshenko to make a statement about being pressured into removing Shokin from his post, then an investigation must be launched.

However, he said, neither his agency nor the NABU had received such a statement thus far.

Likewise, he said his department couldn’t initiate an investigation based solely on comments it had heard from the U.S. side.

“If there was pressure from an official of a foreign country then, first off, the law enforcement agencies of that country must ask what it was.”