Ukraine's prosecutor-general said he would review cases related to a gas firm where Joe Biden's son used to work, after US President Donald Trump pushed his Ukrainian counterpart to look into the issue.

Mr Trump's request that Ukrainian President Volodymyr Zelensky investigate the activities of Hunter and Joe Biden, the frontrunner for the Democratic presidential candidacy, has sparked an impeachment probe in the United States.

Ukrainian Prosecutor-General Ruslan Ryaboshapka told reporters his office would revisit about 15 cases but stressed that they were not necessarily connected to Mr Biden's son, Hunter.

"We are conducting an audit of the cases that were earlier overseen by the Prosecutor-General's Office," said Mr Ryaboshapka.

"We are reviewing all cases that were closed or broken up into separate cases, to make a decision on whether this was illegal."

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Ukrainian authorities have previously said the investigation into gas firm Burisma focuses on a period before Hunter Biden joined the company's board.

Mr Ryaboshapka also suggested the prosecutors may focus not on Hunter Biden but on Burisma's founder Mykola Zlochevsky and other figures.

"As far as we can see, this is more a question of Zlochevsky and (Ukrainian businessman Sergei) Kurchenko than Burisma and Biden," he said.

"This work is continuing and we'll report on its results later," Mr Ryaboshapka added.

Opponents of Mr Trump say the president used promised financial aid as leverage over the Ukrainian leader in a bid to pressure him into investigating Joe and Hunter Biden.

Text messages released by Democrats show Kurt Volker, Mr Trump's former US special envoy to Ukraine, telling a top Zelensky aide that he should open an investigation in return for a visit to Washington.