Republican Donald Trump swept to victory in five north-eastern state primaries Tuesday, while Democrat Hillary Clinton won four states as the front-runners cemented their status in the state-by-state process to pick presidential candidates.

Trump won by large margins in all five states: Maryland, Pennsylvania, Delaware, Connecticut and Rhode Island, with the billionaire real estate mogul nabbing more than 60 per cent of the vote in two of states and well over 50 per cent in the others, official results showed.

Trump said after the victory, comig on the heels of a big win in his homestate of New York a week earlier, that he considers himself the "presumptive nominee" of the Republican Party despite not having yet earned enough convention delegates awarded through the votes to lock up the nomination.

"This is a far bigger win than we ever expected - all five," Trump told supporters at Trump Tower in New York.

Clinton's leads were just as commanding, but she faced a loss to rival Bernie Sanders in the small state of Rhode Island.

"With your help we're going to come back to Philadelphia for the Democratic National Convention with the most votes and the most pledged delegates," Clinton said at a rally in the city that hosts the party's July convention.

"We will unify our party to win this election and build an America where we can all rise together, an America where we lift each other up instead of tearing each other down."

Clinton maintained that there is more that unites Democrats than divides them and instead took aim at Trump on immigration, women's rights and other issues.

Clinton won 59.7 per cent in Delaware, 63.4 per cent in Maryland, 56.6 per cent in Pennsylvania and 51 per cent in Connecticut, while Sanders won with 57.7 per cent in Rhode Island, according to state election officials.

Clinton is coming off a decisive win in New York on April 19. Her wins Tuesday put her on target to win the 2,383 delegates needed to win her party's nomination.

On the Republican side, Trump secured 63.6 per cent of the vote in Rhode Island, 60.8 per cent in Delaware, and 55 per cent in Maryland, election officials said. In Connecticut he secured 58 per cent and in Pennsylvania 57.2 per cent, partial results in those states showed.

Trump's win in Pennsylvania came in the largest state up for grabs Tuesday, but complicated delegate allocation rules mean that only a small portion of the state's Republican 71 delegates will go outright to the winner of the state vote.

Cruz and Kasich have been trying to coordinate in upcoming contests to prevent Trump from securing the 1,237 delegates needed to win the nomination outright at the party convention in July, but Trump downplayed the move, pointing to his "millions more votes."

Neither Kasich nor Cruz can win enough delegates alone in upcoming primaries to secure the nomination before the convention, but each hopes to keep Trump short and force the delegates to negotiate a different choice.

If the "Stop Trump" tactic is successful, it would lead to a brokered convention in which, after the initial voting, the delegates awarded through the primary process are no longer bound to a specific candidate and become free to chose an alternate candidate.

That would mean that, if Trump does not win enough delegates to secure the nomination during the first round of voting, he could lose support in subsequent rounds.

Cruz and Sanders meanwhile were looking forward to states yet to vote with rallies Tuesday evening in Indiana and West Virginia.

Sanders pointed to his campaign's success coming from relative obscurity to seriously challenge the Clinton juggernaut.

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