Microsoft Corp recently alerted one of Democratic presidential candidate Joe Biden's main election campaign advisory firms that it had been targeted by suspected Russian state-backed hackers, according to four people briefed on the matter.

The hacking attempts targeted staff at Washington-based SKDKnickerbocker, a campaign strategy and communications firm working with Biden and other prominent Democrats, over the past two months, the sources said.

Microsoft Corp identified the suspected hacking group as the same set of spies blamed by the U.S. government for breaking into the campaign of Democratic former presidential candidate Hillary Clinton and leaking the emails of her staff, two of the sources said.