Former military chief Benny Gantz received an official mandate on Wednesday to try to form Israel's next government.

After inconclusive elections in April and September, Gantz's nomination marked the first time since 2008 that someone other than Prime Minister Benjamin Netanyahu, 70, has been asked by Israel's president to build a ruling coalition.

In his acceptance speech, Gantz spoke of "national reconciliation" but also described a future government in terms that could exclude the ultra-Orthodox. 

"I promised to establish a liberal unity government, and that, I intend to do," he said, using political shorthand for an administration that pursues a secular agenda, such as opening of businesses on the Jewish Sabbath.

Head of the centrist Blue and White party, Gantz will have 28 days to complete the task assigned by President Reuven Rivlin in a televised ceremony. Failure could lead to a new election, the third within a year. 

"Everyone expects us to bring the political chaos to an absolute end," Gantz said, accepting the nomination.

Rivlin gave Prime Minister Benjamin Netanyahu the chance to form a government first. But the prime minister, who leads the right-wing Likud party, said on Monday he was abandoning the effort.

"In the past weeks I made every effort to bring Benny Gantz to the negotiating table, every effort to establish a broad national government, every effort to avoid another election," said Netanyahu.

He remains caretaker premier until a new government is established. He has been prime minister since 2009, after a first term from 1996 to 1999, making him Israel's longest-serving leader.

Negotiators from Blue and White and Likud planned to meet again on Thursday.