A growing number of US government agencies have been targeted in a sophisticated hack.

The US Treasury and departments of homeland security, state, defence and commerce were attacked, reports say.

SolarWinds Orion, the computer network tool at the source of the breach, said 18,000 of its 300,000 customers might have been affected.

SolarWinds Orion: More US government agencies hacked https://t.co/tR7nYOiRcQ

— BBC News Technology (@BBCTech) December 15, 2020

Many suspect the Russian government is responsible for the attack, but it denied the claims as "baseless".

It is unclear what information has been stolen or exposed in the hack, but the attackers have been monitoring networks since March and were active as recently as Sunday, the Washington Post reports.

The attacks were first revealed by Reuters, identifying breaches at the Treasury and homeland security, the department which manages cyber-security for the US government.

Parts of the defence department were also breached, the New York Times reports, while the Washington Post says that the state department and National Institutes of Health were hacked.

The UK's intelligence agency GCHQ is currently monitoring the situation and has described the compromises as "serious events".

A number of UK government departments and other organisations use SolarWinds but its unclear if they use Orion.

The list of identified victims is expected to grow as more information about the incident emerges.