MOSCOW/KIEV - Ukraine must investigate the activities of Joe Biden's son to establish whether his role in a Ukrainian gas company complied with the country's laws, Mykola Azarov, Ukraine's former prime minister, said in an interview.Hunter Biden's role in the company, Burisma Holdings Limited, is in focus after the White House published a memo showing U.S. President Donald Trump asked his Ukrainian counterpart Volodymyr Zelenskiy in a July phone call to get prosecutors to look into his activities. Zelenskiy agreed. Joe Biden, former U.S. vice president and a contender in next year's U.S. presidential race, has denied using his influence to get Ukraine's prosecutor general fired to prevent him investigating his son's involvement and has said that he and his son have done nothing wrong.Ukraine's National Anti-Corruption Bureau said on Friday it was investigating activity at Burisma between 2010-2012, but that it was not looking into changes to its board in 2014 when Hunter Biden joined.Hunter Biden was a director on Burisma's board from 2014 until at least 2018, according to documents filed by the company in Cyprus where it is registered.Azarov, who was prime minister from 2010-2014, is himself wanted by Ukrainian authorities for alleged abuse of power.He said he was not aware of any evidence suggesting wrongdoing on Hunter Biden's part, but said it was in the Ukrainian public interest to ascertain the legality of his activities.In particular, he said it was important to investigate what Biden had done for Burisma to justify his fee."I think it's essential," Azarov told Reuters in Moscow where he fled after street protests toppled Russia-friendly President Viktor Yanukovich in 2014."Given that this question has been raised there is a real reason for this to be looked into. It's a fact (his directorship and fees) and not made up. It should be investigated so that the "i" s can be dotted and the "t" s crossed."Azarov said the key thing for investigators to establish from a Ukrainian legal viewpoint would be whether Biden's fee was a token one or whether he actually did any work to justify it."If, using his knowledge, he played an active role then there's nothing scandalous about it. But if he was simply on the books and getting money then that could be seen as a violation of the law."Though he said he was deeply skeptical of the allegations, Azarov said he also thought it was right that the circumstances of the general prosecutor's sacking in 2016 be looked into.In a separate interview in Kiev, Pavlo Klimkin, Ukraine's ex-foreign minister, told Reuters he and others were surprised by the White House's decision to release confidential details of Trump's call with Zelenskiy.According to Ukrainian law, such calls are strictly confidential and Zelenskiy could not be compelled to release the Ukrainian transcript or a summary memo, said Klimkin.Klimkin listened in when former Ukrainian President Petro Poroshenko held phone calls with Trump, but said he had never heard Trump raise Biden in those conversations or try to extract a personal favor in return for U.S. military aid."The style and the way in which Trump communicates was probably similar but the content was different. That's all I can say without releasing details," said Klimkin.