Trump, who has in the past said he would not take sides between Israel and the Palestinians, appeared to change tack in his address on Monday.

"The Palestinians must come to the table knowing that the bond between the United States and Israel is unbreakable," the New York billionaire businessman told the annual conference of the American Israel Public Affairs Committee (AIPAC).

"When I become president, the days of treating Israel like a second-class citizen will end on day one," he said.

Trump was speaking as part of an effort to convince establishment Republicans to get on board his bid for the party's presidential nomination.

Speaking earlier at the event, Clinton warned that Trump had a record of inconsistency on the issue.

"We need steady hands, not a president who says he's neutral on Monday, pro-Israel on Tuesday and who knows what on Wednesday, because everything's negotiable," the Democratic frontrunner told nearly 18,000 attendees. "Israel's security is non-negotiable."

Pledge to deport Muslims

Without directly mentioning Trump by name, Clinton said the White House could not afford to be neutral. "Anyone who doesn't understand that has no business being our president," she said.

Clinton also condemned Trump's pledge to deport illegal immigrants and temporarily ban Muslims from entering the United States. "If you see bigotry, oppose it, if you see violence, condemn it, if you see a bully, stand up to him," she said, drawing attention to "dark chapters" in the US past.

"We remember the nearly 1,000 Jews aboard the St. Louis, who were refused entry in 1939 and sent back to Europe. But America should be better than this. And I believe it is our responsibility to say so."

Sanders stays away

Republicans Ted Cruz and John Kasich also addressed the meeting, but Clinton's challenger for the Democratic nomination, Bernie Sanders, declined. The 74-year-old, who is Jewish, cited a heavy campaign schedule.

Establishment Republicans have been reluctant to endorse Trump. Their best chance of derailing his candidacy for the November 8 presidential election would be to prolong the decision-making process until the party's convention in July.

According to the New York Times, Trump has 680 delegates, with Cruz on 423 and Kasich on 143, with each needing 1,237 to win the nomination. Clinton has 1,663 of the Democratic candidates, with the target being 2,383.