A public official charged with helping victims of Japan's nuclear disaster who launched a foul-mouthed Twitter tirade faced calls to quit Thursday.

In a posting on the micro-blogging site Yasuhisa Mizuno said citizen groups demanding measures to protect people against radiation were "lefty shit", a report said.

"I can't help but only feel pity for their lack of intelligence," he wrote, under the handle @jp1tej.

The Tweets, which had been deleted by Thursday, were reported by the Mainichi newspaper.

Reconstruction minister Takumi Nemoto, who oversees the agency employing Mizuno, apologised for the remarks, and hinted action would be taken.

"In terms of reprimand, we want to take this case appropriately based on results of our probe," he said.

Opposition parties rounded on the remarks, and called on Prime Minister Shinzo Abe to sack the official.

"We strongly demand the dismissal of the reconstruction agency councillor who used abusive language on Twitter," Socialist leader Mizuho Fukushima said.

"We request the Abe cabinet and the reconstruction agency build a system to help (victims)."

More that 18,000 people were killed when an enormous tsunami crashed into Japan in March 2011 after a huge undersea earthquake.

The waters swamped cooling systems at the Fukushima Daiichi nuclear plant, sending reactors into meltdown.

Tens of thousands of people were forced to leave their homes by the spreading radiation, with some expected never to be able to return.