Romney's 'birther' jibe upsets Obama campaign
Republican presidential candidate Mitt Romney has referred at a campaign rally to a discredited theory that President Barack Obama was not born in the US.
Speaking near his childhood home in Michigan, Mr Romney joked that no-one had asked to see his birth certificate.
The "birther" remarks drew a swift response from the Obama campaign, who said the candidate had "embraced" the most extreme conservative claims.
Mr Romney is challenging Mr Obama for the White House in November's election.
"No-one's ever asked to see my birth certificate," Mr Romney told a crowd of about 5,000 people at an event in a Detroit suburb.
"They know that this is the place that we were born and raised," he said, to laughter from the crowd.
The comments allude to a debunked conspiracy theory that Mr Obama, whose father was from Kenya, was not born in the US and is not eligible to be president.
Some conservatives, including real estate mogul Donald Trump, have endorsed the claims, even though Mr Obama has released several copies of his birth certificate and Hawaiian officials have confirmed he was born there.
An Obama campaign spokesman said: "Throughout this campaign, Governor Romney has embraced the most strident voices in his party instead of standing up to them.
"Governor Romney's decision to directly enlist himself in the birther movement should give pause to any rational voter across America."
The Romney campaign sought to put the comments into context: "The governor has always said, and has repeatedly said, he believes the president was born here in the United States.
"He was only referencing that Michigan, where he is campaigning today, is the state where he himself was born and raised."
Mr Romney is due to be officially appointed next week as the Republican nominee at the party's convention in Tampa, Florida.
Also on Friday, former presidential candidate Rick Santorum said he would release to Mr Romney the delegates he amassed during this year's Republican votes to pick the party's White House candidate.
It was the second time in as many days that Mr Romney found himself under fire for comments on the campaign trial.
On Thursday, he said in Minnesota that "big business is doing fine in many places", partly because they benefited from offshore tax havens.
Mr Romney has been lambasting President Obama for saying earlier this year that the "private sector is doing fine".
The Republican's remarks were also a reminder that he himself has kept some of his personal fortune in low-tax foreign accounts.