Republican presidential candidate Donald Trump is pinning the damaging and vandalizing of a local Republican Party office in North Carolina on the supporters of his presidential rival Hillary Clinton and her Democratic Party.

A news release from the town of Hillsborough, N.C., said someone threw a bottle filled with flammable liquid through the window of the Orange County Republican Party headquarters overnight. The substance ignited and damaged furniture and the interior before burning out.

The news release says an adjacent building was spray-painted with the words: "Nazi Republicans leave town or else."

State Republican director Dallas Woodhouse said no one was injured, but a security alert is being sent to party offices around the state.

State Republican director Dallas Woodhouse said no one was injured, but a security alert is being sent to party offices around the state. (Jonathan Drew/Associated Press)

On Sunday afternoon, the walls of the multi-room office were covered in black char, and a couch against one wall had been burned down to its springs. Shattered glass covered the floor, and melted campaign yard signs showed warped lettering. The graffiti had been covered in paint by late afternoon.

Trump tweeted on Sunday, blaming the incident on "animals representing Hillary Clinton and [Democrats] in North Carolina." He also encouraged local Republicans, saying: "With you all the way, will never forget. Now we have to win. Proud of you all!"

Also on Twitter, Clinton called the attack "horrific and unacceptable" and said she was "very grateful that everyone is safe."

Animals representing Hillary Clinton and Dems in North Carolina just firebombed our office in Orange County because we are winning <a href="https://twitter.com/NCGOP">@NCGOP</a> —@realDonaldTrump

The attack on the Orange County HQ <a href="https://twitter.com/NCGOP">@NCGOP</a> office is horrific and unacceptable. Very grateful that everyone is safe. —@HillaryClinton

State Republican executive director Dallas Woodhouse said people sometimes work after-hours, and he felt lucky that no one was there at the time. He said the bottle appeared to have landed on or near the couch where volunteers sometimes take naps.

"They are working around the clock. It is a miracle that nobody was killed," he said in an interview, calling the fire "political terrorism."

He said Republican offices around the state were re-examining their security.

The violent act in the key battleground state was condemned by public figures across the political spectrum.

At a news conference, Woodhouse urged Republicans to respond peacefully by turning out to vote in November. He said he'd received messages of support from Democrats.