"We have seen multiple intelligence experts as well as security firms describe the cyber attack on the DNC [Democratic National Committee] as carried out or conducted by Russian government-sponsored entities," Sullivan told reporters at the Democratic National Convention.
Earlier on Monday, the Democratic National Committee said in a statement that the party is working with the US Federal Bureau of Investigation (FBI) to determine the size and scope of the breach.
Sullivan claimed that if Russia is behind the leak, it would be a new "watershed."
"This would be Russia interfering in the American presidential election," he stated.
Kremlin spokesman Dmitry Peskov declined to comment on the allegations on Monday, but has earlier stated that a possibility of the Russian government’s involvement in DNC cyber attack is completely ruled out.