Twitter on Friday flagged as premature posts referring to Joe Biden as "president-elect," as the vote count continued in the knife-edge US election with the Democrat leading Donald Trump in several key states.

Tweets referring to the former vice president with the victor's title and his running mate Kamala Harris as "vice president-elect" were tagged with messages saying counts were not yet final.

"Official sources may not have called the race when this was tweeted," read a Twitter message below a post from Democratic Coalition co-founder and podcaster Scott Dworkin using the two titles for Biden and Harris.

The notice came with a link to information about the status of the election.

Twitter told AFP that the move was in line with its Civic Integrity Policy and in keeping with the position it has taken since the polls closed late Tuesday.

"We will label Tweets claiming victory that meet our criteria as both Pennsylvania and the presidential race overall have only been called by one source," a Twitter spokesman said.

"Per our rules, until a second source makes this determination, claims of victory must cite a source that has made the call."

Twitter accounts that qualify for posts to be labeled include presidential campaigns or contenders, as well as US-based accounts with more than 100,000 followers or tweets being massively liked or quoted.

Voting results will be consider official by Twitter when announced by state election officials or when calls about an outcome are backed by at least two national news outlets with independent decision desks, according to the one-to-many messaging service.

Twitter and Facebook have been scrambling to flag, mask and limit the spread of premature claims of victory or false attacks on the voting process since the polls closed late Tuesday.

"As votes are still being counted across the country, our teams continue to take enforcement action on tweets that prematurely declare victory or contain misleading information about the election broadly," Twitter said.

Unfounded claims by Trump regarding the voting process as well as premature claims of victory about either candidate in the race have been flagged or masked, with links provided to reliable sources of information.

Twitter's action made the comments less visible, and users seeking to read the posts were required to click through a warning.