An atheist rights group in the US has erected a number of billboards across two states urging people not to bother attending church this Christmas.

The boards show a Santa figure next to the words "Go ahead and skip church! Just be good for goodness' sake. Happy holidays!"

They have been placed in Winston-Salem, North Carolina and Colorado Springs, Colorado.

"We want people to know that going to church has absolutely nothing to do with being a good person," said David Silverman, president of American Atheists which is behind the ads.

"The things that are most important during the holiday season – spending time with loved ones, charity, and being merry – have nothing to do with religion."

A billboard erected by the organisation in 2014 showed a child writing a letter to Santa, telling him she didn't want to go to church.

"This year, Santa wrote back," Silverman said.

The Pew Research Centre recently found that fewer Americans are now "absolutely certain" that God exists.

Overall, the American public is becoming less religious – but only slightly. Pew's Religious Landscape Study last year found that 3.1 per cent of American adults consider themselves atheists; an increase from 1.6 per cent in 2007.

A number of atheists still go to church, however. According to Pew, one per cent of atheists attend church weekly, and one in ten go at least once a year. One per cent of atheists say they pray daily or more often, compared to two thirds (68 per cent) of Christians.

American Atheists said the billboards have been designed to reach people who identify as Christian, but have doubts about their beliefs.

"It is important for these folks who are on the fence about their beliefs to know that they can take that first big step and leave church," national program director, Nick Fish, said.

"There are tens of millions of atheists in this country. We're everywhere. And we don't need church or gods to tell us how to be good people."