"There is no evidence that any voting system deleted or lost votes, changed votes, or was in any way compromised," US election officials said.
Winning Pennsylvania’s 20 electoral votes would put the former vice president over the 270 he needs to secure the presidency.
Democrat Joe Biden said he was confident of victory and President Donald Trump accused Democrats of trying to steal the election.
The series of election results has commenced from different states for one of the most polarising presidential election in US history.
Twitter said in a blog post that, from next week, users will get a prompt pointing them to credible information before they can retweet content that has been labeled as misleading.
Facebook Inc said on Tuesday it would affix labels to political ads shared by users on their own feeds
Facebook has been accused of helping Trump win through misinformation that was posted by foreign governments online.
Sanders, 78, a democratic socialist U.S. senator from Vermont, is considered the front-runner for the Democratic nomination and is favored to win the Nevada caucuses on Saturday.
Donald Trump and Russia both angrily dismissed US intelligence's account that Moscow is meddling in this year's election as Democrats accused the president of betraying democracy.