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The Ukrainian gas company at the centre of President Donald Trump's impeachment was successfully hacked by Russian military agents in November, a US cybersecurity firm has said.

Area 1 Security said it was not clear what the hackers were searching for when they hacked Burisma Holdings, according to BBC News.

Democratic presidential contender Joe Biden's son Hunter Biden was previously on the board of the firm.

Burisma, the US government and Russia's Ministry of Defence are yet to respond.

Area 1 linked the hack - a "phishing" attack - to Russia's military intelligence unit known commonly as the GRU.

President Trump was impeached in December over allegations that he improperly pressured Ukraine's leader Volodymyr Zelensky to launch an investigation into Burisma and its links to the Bidens.

The security firm also drew parallels between November's hack and another that was carried out against Democrats in 2016, before the US presidential election.

In its eight-page report, California-based firm Area 1 said the GRU hackers behind the Burisma breach are known by the alias "Fancy Bear" among cybersecurity researchers.

The hackers used "phishing" emails that are designed to steal usernames and passwords, the California-based security firm said.

In this case, Burisma employees were sent what looked like internal company emails and fake websites that looked like the sign-in pages of Burisma subsidiaries.

Using this tactic, the hackers managed to get into Burisma's servers, it added.

Area 1 said it became aware of the hack after its email security scanning product found suspicious evidence online, including "decoy domains" for the fake websites.

A source close to Burisma told Reuters news agency that the company's website had been subject to a number of break-in attempts over the past six months.