Rick Santorum has won the US Republican primaries in Alabama and Mississippi, US networks have projected.

In Alabama, Mr Santorum was on 35%, with Newt Gingrich in second place and Mitt Romney trailing in third.

The Mississippi race was closer, but Mr Romney appeared set to lose out to Mr Gingrich for second place.

Despite his strong showing Mr Gingrich is expected to face calls for him to stand aside and let Mr Santorum take on Mr Romney in upcoming contests.

However, Mr Gingrich appeared defiant in his concession speech, saying that he would keep on accumulating delegates all the way until the Republican convention in Tampa this August. "We're going to go all the way to Tampa to compete for the nomination," he told supporters in Birmingham, Alabama.

Mr Santorum addressed exultant supporters in Louisiana, which votes next week, saying: "We did it again."

"We will compete everywhere," he added.

"The time is now for conservatives to pull together. The time is now to make sure that we have the chance to win the election and the best way to win the election is to nominate a conservative to go up against Barack Obama so we can take him on on every issue."

Exit polls appeared to project a stronger showing for Mr Romney, who had seen his prospects of a win in at least one of the two state rise in recent days.

The fourth candidate, Texas Congressman Ron Paul, who has not campaigned actively in either state, is trailing far behind with single figure returns.

Romney's 'away game'

Mr Romney, a former Massachusetts governor, ignored his rivals on Tuesday, instead focusing attacks on Mr Obama.

Campaigning in Missouri, which holds caucuses this Saturday, he told supporters Mr Obama had failed to take responsibility for unemployment and was not bothered by high petrol prices.

Mr Santorum won the vote of Alabama Governor Robert Bentley, but could not secure his pre-primary endorsement.

As spokesman said Mr Bentley voted for Mr Santorum in his hometown of Tuscaloosa because the governor considers him "the most conservative candidate in the Republican presidential race".

The Romney campaign argues that his rivals cannot catch him in the hunt for delegates needed to become the Republican nominee, who will challenge Mr Obama for the White House in November.

Mr Santorum's campaign has appeared to accept that in recent days, with the candidate and his aides telling the media that they are now aiming to prevent Mr Romney reaching the winning post.

That would force a meaningful vote at the Republican convention this summer that Mr Santorum believes he would win, according to reports.

Mr Romney said last week that the Deep South contests were "a bit of an away game".

The Harvard-educated north-easterner has been accused of pandering to Mississippi and Alabama voters by addressing them as "y'all" at rallies and singing the praises of grits, a popular local corn-based dish.

He has been endorsed by the Republican Governor of Mississippi, Phil Bryant, and well-financed backers have spent twice the combined sum that Mr Gingrich and Mr Santorum's allies have invested in Tuesday's primaries.

Correspondents say Mr Romney, a Mormon, could struggle to win over evangelical Christians, a key element of the Republican base in the Deep South.

He is also viewed with suspicion because he was governor of Massachusetts, a liberal state. But he has benefited from the splitting of the anti-Romney conservative vote between Mr Santorum and Mr Gingrich.

Mr Santorum, a former Pennsylvania senator, is hoping to channel his appeal with social conservatives to score a knockout blow to Mr Gingrich's hopes for a Southern comeback.

"People of Mississippi and Alabama want a conservative," Mr Santorum said on the eve of voting. "If they want a conservative nominee for sure, they can do that by lining up behind us and making this race clearly a two-person race outside of the South."

But Mr Romney told Fox News that Mr Santorum would need to "pull off a miracle" to overtake him in the delegate count.

And Mr Gingrich, campaigning in Birmingham, Alabama, did not seem ready to stand aside.

"I've stayed in this race for two reasons," he said. "I do not believe the two other candidates can beat President Obama."

Approval indications

Mr Romney, who is the Republican establishment's favourite, has now won 17 of the 26 state or territory votes, compared to seven wins for Mr Santorum.

Mr Gingrich, a former House of Representatives Speaker, has only won two contests, both in the South, including his home state of Georgia.

Alabama and Mississippi together have 90 delegates, and both states award them proportionally.

Hawaii and American Samoa also hold caucuses on Tuesday.

The votes take place as two surveys indicated a fall in approval for President Obama, amid anger at escalating petrol prices.

A Washington Post/ABC News poll found that 46% of those surveyed are happy with the president's handling of his job, and 50% disapprove.

A New York Times/CBS poll found 41% approval, and 47% disapproval.

But a third poll released on Tuesday evening offered good news for the White House: the Reuters/Ipsos survey put Mr Obama's approval rating at 50%, with 48% disapproving.