Donald Trump's rivals for the Republican nomination have announced a surprise deal with the aim of blocking the controversial billionaire who leads the polls.

In an extraordinary move both Ted Cruz's and John Kasich's campaign issued statements that outlined the pair would not compete with each other in upcoming primaries. The unprecedented tactics aim to ensure the Republican candidate "can unify the Republican party and win in November," according to Cruz's statement.

"Our campaign will focus its time and resources in Indiana and in turn clear the path for Governor Kasich to compete in Oregon and New Mexico, and we would hope that allies of both campaigns would follow our lead," said Cruz's campaign manager Jeff Roe.

Kasich's announcement quickly followed: "We are very comfortable with our delegate position in Indiana already, and given the current dynamics of the primary there, we will shift our campaign's resources West and give the Cruz campaign a clear path to Indiana," said Kasich campaign chief strategist John Weaver.

Trump blasted the alliance as a "horrible act of desperation". In a statement he said: "It is sad that two grown politicians have to collude against one person who has only been a politician for ten months in order to try and stop that person from getting the Republican nomination."

Trump has won the most state nominating contests, but he has a tough path to earn the 1,237 delegates needed to secure the nomination. The Cruz and Kasich campaigns believe their agreement to cede states where the other candidate appears strong could help limit Trump's ability to win more delegates.

Some Republican strategists who oppose Trump have been calling for such a deal for weeks. The question for Cruz and Kasich is whether their agreement is too late.

If no candidate has enough support by the first vote at the Republican National Convention in July, many delegates will be allowed to switch sides on subsequent ballots.

Cruz campaign manager Jeff Roe said Trump, who has offended women, Hispanics and other groups with controversial statements, would lose a general election contest against the eventual Democratic nominee in the November 8 election.

"Our goal is to have an open convention in Cleveland, where we are confident a candidate capable of uniting the party and winning in November will emerge as the nominee," Weaver said in a statement.

Late on Sunday, Trump tweeted his reaction:

Trump has frequently complained that the Republican Party's nominating process is "rigged" against him because establishment party members oppose him. Party officials argue that the delegate selection rules have been known for some time.

The stop-Trump group #NeverTrump welcomed news of the pact.

"Whether you support Ted Cruz or John Kasich, a second ballot at the Convention is imperative to stopping Donald Trump. We're happy to see the Kasich and Cruz campaigns strategically using their resources to deny Donald Trump delegates where they are in the strongest position to do so," said the group's senior adviser, Rory Cooper.

The Indiana primary is on May 3, Oregon's is May 17 and New Mexico's June 7.

Additional reporting by Reuters.