Police tightened security around the presidential office Cheong Wa Dae on Sunday after an unidentified man who police presume made bomb threats on Twitter weeks ago called in another threat, officials said.

The man, apparently calling from an international phone number, dialed Cheong Wa Dae's civil affairs office five times since 2:39 a.m., according to police.

The man said he would bomb the office "if you don't show willingness by noon," police said, adding it was unclear what he meant by "willingness."

Though the caller ID showed the country code of France, police were continuing to trace the phone number since numbers could be altered.

An initial survey around the office found no explosive devices, police said, adding security forces have been beefed up to prepare for contingencies.

South Korea's anti-terrorism officials, meanwhile, said the caller appeared to have made similar threats on Twitter two weeks ago.

Authorities said the man may have been the one who tweeted warnings to bomb the private homes of President Park Geun-hye and her chief of staff Kim Ki-choon on Jan. 17.

Police have earlier traced the twitter account to a South Korean man in his 20s living in France.

They will ask French authorities for cooperation and try to have him extradited, police said, adding he will face charges of obstruction of justice if repatriated. (Yonhap)