The Russian spy agency has said telephones belonging to foreign diplomats based in Russia and the former Soviet Union, including those from Israel, Syria, China and Nato members, had been targeted.

Snowden fled the US and was given asylum in Russia after leaking secret files in 2013 that revealed vast domestic and international surveillance operations carried out by the NSA, where he worked.

Microsoft, one of more than 100 companies attacked and 18,000 left vulnerable by the hack, has analysed the work it took to insert malware into widely used security software created by SolarWinds.

Intelligence work often must be done in special locations which are protected from outside eavesdropping and cannot be done from home.