About 500 ballots were also discovered in North Carolina during sweeps
The US Postal Service (USPS) said about 1,700 ballots had been identified in Pennsylvania at processing facilities during two sweeps Thursday and were being delivered to election officials.
In a court filing early Friday, USPS said 1,076 ballots, had been found at the USPS Philadelphia Processing and Distribution Center. About 300 were found at the Pittsburgh processing center, 266 at a Lehigh Valley facility and others found at other Pennsylvania processing centers.
Ballots must be received by Friday evening in Pennsylvania in order to be counted. The vote for the US president remains extremely close and Pennsylvania is one of the states that remains undecided.
About 500 ballots were also discovered in North Carolina during sweeps, USPS said on Friday.
US District Judge Emmet Sullivan on Thursday had ordered twice daily sweeps at USPS facilities serving states with extended ballot receipt deadlines as votes were still being counted in US election battleground states.
Some states, including Nevada and North Carolina, are counting ballots that are received after Election Day as long as they were postmarked by Tuesday.
Lawyers said at a court hearing on Thursday that USPS had delivered about 150,000 ballots on Wednesday.
"The vast majority were destined for postmark states and would be delivered on-time under state election law," USPS said.
Sullivan said the processing centers must perform morning sweeps and then afternoon sweeps "to ensure that any identified local ballots can be delivered that day."
Sullivan issued a separate order requiring USPS to "coordinate with all local county Boards of Elections in North Carolina or Pennsylvania" in order to deliver all ballots "before 5:00 PM local time in North Carolina or Pennsylvania" on Friday.
Ballots were still being counted by election officials in battleground states after polls closed Tuesday in one of the most unusual elections in US history because of the novel coronavirus pandemic.
Democratic candidate Joe Biden was cutting sharply into Republican President Donald Trump's leads in Pennsylvania and Georgia. The former vice president retained slim margins in Nevada and Arizona.