US President Joe Biden sought to calm a divided nation after his rival Donald Trump survived an assassination bid, saying in a rare Oval Office address on Sunday it was time to lower the temperature of America's hostile politics.

"You know, the political record in this country has gotten very heated. It's time to cool it down," Biden said in the televised speech, just the third he has given from behind the historic Resolute Desk during his presidency.

As the country reeled from images of a bloodied Trump waving his fist after a gunman opened fire at a rally in Pennsylvania, Biden added that US politics "must never be a literal battlefield, a, God forbid, a killing field."

The 81-year-old Democrat said that both sides had a responsibility to ease the situation ahead of a deeply divided election that would now be a "time of testing" for the United States.

Investigators said that they were still probing the motives of 20-year-old gunman Thomas Matthew Crooks, who was shot dead by snipers at the event in Butler, Pennsylvania, on Saturday after firing multiple shots with a legally-bought semi-automatic rifle.

Former president Trump said on social media on Sunday that Americans should not allow "evil to win," adding it was "more important than ever that we stand United." (AFP)