WASHINGTON: More than one in three conservative Republican voters still thinks US President Barack Obama is a Muslim, nearly four years after he won power, said a Pew Research Centre survey Thursday.

“The number of Americans who say Obama is a Muslim is up since 2008, similar to 2010,” senior Pew researcher Greg Smith told AFP.

Although the number of conservative Republicans who believe Obama is a Muslim stagnated between 2010 and 2012, it has more than doubled since 2008 despite the Christian church-goer's repeated affirmations of his faith.

Seventeen per cent of 3,000 overall voters asked by Pew in June and July thought Obama was Muslim, the poll said. That number is up five per cent from 2008 and down three per cent from 2010, within the survey's margin of error.

“Our polling suggests there's a real partisan component,” Smith said.

Muslim American community leaders expressed concern that the lingering inability to recognize Obama's true faith showed a disturbing undercurrent in US politics, indicating a possible rise of Islamophobic discourse.

“For 17 per cent of people to believe Obama is a Muslim shows there's a lot of fear-mongering and politicking in America,” Haris Tarin from the Muslim Public Affairs Council.

But Tarin opposes Obama's efforts to “constantly go out of his way to say he's a Christian.”

Obama's Kenyan father was born a Muslim, his middle name is Hussein, and he spent years in Muslim-majority Indonesia as a child.

Millions of Americans — particularly Republicans fuelled by conservative bloggers and talk radio hosts — suspect he is secretly Muslim, according to several polls.

“My Christian faith then has been a sustaining force for me over these last few years, all the more so when Michelle and I hear our faith questioned from time to time,” Obama said at the 2011 National Prayer Breakfast, an annual gathering that brings together thousands of religious and community leaders.

Tarin, the director of the Washington office of the Muslim American advocacy group, however said he hoped the poll would not lead to another Obama assertion of faith.

“I hope Obama does not go out of his way to deny he's a Muslim. That reinforces the idea that Islam can be used as a slur,” said Tarin.

Tarin noted that former secretary of state under the Republican George W Bush administration, Colin Powell, dealt a blow to the controversy of Obama's religion, when he asked in an interview on NBC's Meet The Press, “What if Obama is” a Muslim?

Only 49 per cent of those polled by Pew correctly identified Obama as a Christian, while another 31 per cent said they did not know.

Some 60 per cent of voters correctly identified Obama's Republican opponent in the November presidential elections, Mitt Romney, as a Mormon.