Hackers aligned with Russia's government breached US Democratic National Committee computers and stole data including a trove of opposition research on Republican presidential hopeful Donald Trump, the Washington Post reported Tuesday.

The cyber-attack was so comprehensive, including one access to the DNC network that lasted the course of an entire year, that the intruders were able to read all email and chat traffic on the committee's system, DNC officials and security experts said, according to the daily.

A computer cleanup operation this past weekend expelled the hackers from the system, the officials and experts said, and no financial, donor or personal information appeared to have been compromised, the Post said.

The security firm that conducted the cleanup, CrowdStrike, said Tuesday on its website that they received a call from the DNC to respond to a suspected breach uncovered in April.

The firm said it quickly identified "two sophisticated adversaries on the network" including COZY BEAR, which CrowdStrike said successfully hacked into unclassified networks of the White House and State Department.

"Both adversaries engage in extensive political and economic espionage for the benefit of the government of the Russian Federation and are believed to be closely linked to the Russian government's powerful and highly capable intelligence services," the firm said.

As a main US adversary, Moscow is undoubtedly interested in obtaining information about potential future American leaders and their policies and weaknesses.

Russian hackers have also targeted the campaigns of Trump and his Democratic rival Hillary Clinton, as well as some Republican political action committees, the Post reported, citing US officials.

"The security of our system is critical to our operation and to the confidence of the campaigns and state parties we work with," House Democrat and DNC chair Debbie Wasserman Schultz said in a statement to the paper.

DNC and CrowdStrike teams "moved as quickly as possible to kick out the intruders and secure our network," she said.