Election in Israel: Blue and White party leader Benny Gantz at the party's headquarters. (Reuters)

Prime Minister Benjamin Netanyahu's main election challenger, centrist party chief Benny Gantz, said on Wednesday it appeared from exit polls that the Israeli leader was defeated but that only official results would tell.

Gantz, a former general, stopped short of an outright claim of victory. But beaming confidence, he told a rally of his Blue and White Party that it appeared "we fulfilled our mission", and he pledged to work towards formation of a national unity government.

Netanyahu, he said, apparently "did not succeed in his mission" to win a fifth term in a do-over election that followed an inconclusive national ballot in April. "We will await the actual results," Gantz said.

The prime minister, head of the right-wing Likud and Israel's longest-serving leader, was due to deliver a "brief but important" address at its election headquarters at around 3 a.m. (0000 GMT).

Revised surveys, several hours after polls closed, by Israeli TV stations gave Likud 30 to 33 of parliament's 120 seats, a slight drop from earlier forecasts, versus 32 to 34 for Blue and White.

Neither had enough support, at first glance, for a governing coalition of 61 legislators, and Netanyahu's ally-turned-rival, former Defence Minister Avigdor Lieberman, emerged as a likely kingmaker as head of the far-right Yisrael Beitenu party.

"Netanyahu has lost, but Gantz hasn't won," said Udi Segal, a prominent Israeli television news anchor.

The revised polls showed that without Yisrael Beitenu's projected eight to nine seats, stalemate could ensue: Likud would have the support of only up to 55 legislators, down from 57 in the earlier exit polls, for a right-wing coalition. Blue and White could enlist the backing of no more than 59 for a centre-left government.

"We have only one option - a national, liberal, broad government comprising Israel Beitenu, Likud and Blue and White," said Lieberman, whose projected tally was double the result in April.

Coalition-building could be complicated: Lieberman has said he would not join an alliance that included ultra-Orthodox parties - Netanyahu's traditional partners.

Gantz has ruled out participating in an administration with Netanyahu, if the Israeli leader is indicted on looming corruption charges.

CORRUPTION CHARGES

Dubbed "King Bibi" by his supporters, Netanyahu, 69, had already been stung by his failure to form a government after the April poll.

Looming corruption charges - he has denied any wrongdoing - have also chipped away at Netanyahu's seeming invincibility, 10 years into consecutive terms as prime minister marked by a sharp focus on security that resonated with voters.

"Unless that miraculous turnabout between the exit polls and the actual results happens - the Netanyahu magic has been broken," Anshel Pfeffer, author of a Netanyahu biography, wrote in the left-wing Haaretz daily.

Addressing supporters after the exit polls, Lieberman appealed to President Reuven Rivlin to invite Netanyahu and Gantz to meet as early as Friday, even before final results are in, to explore the formation of a national unity government.

The two main parties' campaigns pointed to only narrow differences on many important issues: the regional struggle against Iran, the Palestinian conflict, relations with the United States and the economy.

An end to the Netanyahu era would be unlikely to bring about a significant change in policy on hotly disputed issues in the peace process with the Palestinians that collapsed five years ago.

ANNEXATION

Netanyahu has announced his intention to annex the Jordan Valley in the occupied West Bank, where the Palestinians seek statehood.

But Blue and White has also said it would strengthen Jewish settlement blocs in the West Bank, with the Jordan Valley as Israel's "eastern security border".

As in the election five months ago, Netanyahu's opponents, including Gantz, focussed on bribery and fraud allegations against the prime minister in three corruption cases.

Netanyahu is due to face a pre-trial hearing in October to argue against the charges being filed.

An election loss could leave him more at risk of prosecution, without the shield of parliamentary immunity that his political allies had promised to seek for him. There is no certainty they would stand by a weakened leader without an obvious public mandate in any coalition-building.

Netanyahu portrays Gantz, 60, as inexperienced and incapable of commanding respect from world leaders such as US President Donald Trump.

Before the last election, Trump gave Netanyahu a lift with US recognition of Israeli sovereignty over the Golan Heights, captured from Syria in the 1967 Middle East war. This time, the White House seems more preoccupied with Iran.

The Trump administration plans soon to release an Israeli-Palestinian peace plan that may prove a dead letter: The Palestinians have rejected it in advance as biased. Netanyahu's open door in Washington and other world capitals, at a combustible time on Israel's borders with Syria, Gaza and Lebanon, remains a big draw domestically.

In the final hours of campaigning, Netanyahu strained every sinew, urging voters to support him to avert what he described as the "disaster" of a left-wing government.

His voice hoarse, the veteran leader took to the streets and social media, at one point using a megaphone in Jerusalem's bus station, to appeal to voters to extend his unbroken decade in power.

In Gaza, Palestinians awaited the results of the vote.

"This election affects many things in our life," said Mohamad Abdul Hay Hasaneen, a janitor in the city of Khan Younis. "There might be limited escalations after the election, but I don't think this would result in a full war."