Donald Trump wins all five Northeast primaries

Billionaire businessman Donald Trump scored wins in all five northeastern US states holding presidential primaries Tuesday - a clean sweep for the Republican frontrunner that propelled him closer to clinching the party's nomination.

Trump defeated his rivals Ted Cruz and John Kasich in Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island.

Hillary Clinton was dominant in Democratic contests in Delaware, Maryland, Pennsylvania and Connecticut. She ceded Rhode Island to rival Bernie Sanders.

"Thank you, Maryland," Clinton tweeted almost immediately after polls closed at 8:00 pm (0000 GMT) in the state that borders the capital Washington.

Should Clinton's performance match Trump's victories, she will push Sanders to the brink of defeat and lift herself to the cusp of a historic nomination.

Trump's big night extended his lead in the all-important race to win the delegates who will officially choose the Republican nominee at their convention in July.

Trump's early triumph comes in the heated aftermath of the revelation that Cruz and Kasich, desperate to prevent the frontrunner from securing the nomination before the convention, were teaming up to block him in future primary races.

Kasich agreed to forego campaigning in Indiana, a winner-take-all state that votes May 3, and Cruz will return the favor later in New Mexico and Oregon.

Sanders has deflected recent questions about whether he would actively support a Clinton candidacy if she is the nominee, suggesting it was up to her to win over his passionate young followers.

But within hours of the surprise deal, Kasich - the governor of Ohio - was already playing it down, saying he was not telling his supporters in Indiana not to vote for him.

"This joke of a deal is falling apart, not being honoured and almost dead," Trump said. "Very dumb!"

Like Trump, Clinton was favored to win all five contests, with polls giving her a double-digit lead over Sanders in Pennsylvania, the biggest state of the bunch with 189 delegates.

Should she run the board, it would heap pressure on Sanders, who has vowed to fight on until the California primary on June 7.

"I don't accept there is no path forward. Let's not count our chickens before they're hatched," Sanders told MSNBC Tuesday.

Sanders has deflected recent questions about whether he would actively support a Clinton candidacy if she is the nominee, suggesting it was up to her to win over his passionate young followers.

Convention hopes

Cruz's campaign had acknowledged that the US senator from Texas was not expecting a perfect night Tuesday, and the candidate sounded eager to move on to what he called "more favorable terrain."

"Tonight this campaign moves back to Indiana, and Nebraska... and Washington and California," Cruz told supporters in Indiana.

According to a recent CBS poll, Trump leads Indiana with 40 percent of likely Republican voters, compared to 35 percent for Cruz and 20 percent for Kasich.

Losing Indiana would make it much harder for Trump to gain the 1,237 delegates needed to win the nomination in the first round of balloting at the party's convention in Cleveland on July 18-21.

Party heavyweights, alarmed by the prospect of a Trump nomination, have long pressed for a united effort around a single candidate against him.

If he falls short, Trump runs the risk that his delegates, most of whom are bound to vote for him in only the first round, will desert him in subsequent rounds.

Cruz and Kasich have openly said they are counting on a contested convention, where they have a shot at wooing enough delegates to snatch the nomination.

Cruz in particular has been successfully maneuvering in state party conventions to have individuals named to delegate slots who, though initially bound to Trump, would be sympathetic to Cruz in later rounds once free to vote for whomever they choose.

Party heavyweights, alarmed by the prospect of a Trump nomination, have long pressed for a united effort around a single candidate against him.

But Cruz is almost as unpopular with the party's establishment as Trump, and Kasich has refused to bow out even though he has only won his home state of Ohio.