US Republican White House contender Mitt Romney has pulled off a double win in the Michigan and Arizona primaries.

"We didn't win by a lot, but we won by enough and that's what counts," Mr Romney said in Michigan.

In Arizona, he leads with 47% of votes, and heads Rick Santorum in Michigan by 41% to 38% with 99% counted.

Michigan was seen as vital for Mr Romney, born in the state where his father was governor, but had struggled to win over conservative voters.

Rick Santorum addressed supporters in Michigan, moments after he reportedly called Mr Romney to concede defeat, thanking voters for their support.

Former House Speaker Newt Gingrich and Texas Congressman Ron Paul did not compete in Arizona and Michigan, focusing on contests scheduled for next week.