"We don't have full insight into the scope of the compromise", said Jeanette Manfra, a cyber security official for the U.S. Department of Homeland Security.
Separately, the White House has said American objectives in Syria have not altered, reiterating that President Donald Trump wants United States troops home "as quickly as possible". "It is a tremendous weapon in the hands of an adversary", he told journalists on a conference call.
The United Kingdom and the United States have accused Russian Federation of instigating a malicious cyber-attack, which affected "millions of machines", including those in hundreds Australian businesses.
"Whether an end user, private firm or device designer, there are straightforward measure we can take to make sure we get network security right, which includes building devices from the ground up to be secure by design because we have seen in this case default passwords and unsecured devices being exploited", he said.More news: Southampton 2 Chelsea 3: Two-goal substitute Giroud inspires turnaround
Some private-sector cyber security experts have criticized the USA government for being too slow to release information about cyber attacks.
Russia has heavily criticized the US, UK and France for launching airstrikes against Russia-ally Syria, while Washington and London have accused Moscow of responsibility for a nerve-agent attack against a former Russian double agent in southern England.
Ciaran Martin, CEO of the UK National Cyber Security Centre said: "Russia is our most capable hostile adversary in cyber space so dealing with their attacks is a major priority for the National Cyber Security Centre and our USA allies. We can't rule out Russian Federation may attempt to use this [hacked] infrastructure for further attacks". It marks an important step in our fight back against state-sponsored aggression in cyber space.
Australia later joined forces with the United Kingdom and USA, saying 400 of their companies had also been caught up in Russian state-sponsored attacks that were identified in August.More news: 2 bodies, vehicle found after California family's SUV went into river
The accusations come at a time of dramatically increased tensions between Russian Federation and the west. Moscow has denied previous accusations that it carried out cyber attacks on the United States and other countries.
He said the UK, Nato and European Union had closed the normal channels of communication with Russian Federation which provided safeguards against confrontation.
"They could be pre-positioning for use in times of tension", said Ciaran Martin, chief executive of the British government's National Cyber Security Centre cyber defence agency, who added that "millions of machines" were targeted.
"The UK will continue to respond to malicious cyber activity in conjunction with global partners such as the United States". "We condemn this latest activity in the strongest possible terms". "We are evaluating but nothing to announce right now", she said.More news: GE3, the massive asteroid that grazed the Earth on Sunday
And shadow attorney general Baroness Chakrabarti questioned the Government's justification for the airstrikes, telling BBC Radio 4's Today programme: "You can't use force under worldwide law just to punish Syria for bad behaviour".