Ex-military chief Gantz was given the mandate to form a government four weeks ago by President Reuven Rivlin.
Jerusalem: Israel's president on Sunday rejected parliament speaker Benny Gantz's request for more time to form a government, as talks persist on a possible interim alliance with Prime Minister Benjamin Netanyahu.
Ex-military chief Gantz was given the mandate to form a government four weeks ago by President Reuven Rivlin following Israel's March 2 election, the country's third inconclusive vote in a year.
Then in a surprise move last month, Gantz was elected speaker of Israel's parliament, or Knesset, and pledged to seek an emergency unity alliance with Netanyahu to ease the political deadlock and help the country tackle the coronavirus pandemic.
While his mandate to form a government runs until the end of Monday, Gantz was no longer actively engaged in talks to forge a coalition led by him as prime minister.
Instead, he was widely believed to be pursuing a unity deal that would see Netanyahu remain as premier for a defined period, possibly then handing power to Gantz.
Despite reports of progress from the two sides -- and from Israeli media -- no deal has been agreed.
Late Saturday, Gantz asked Rivlin for an extension of his mandate.
In response, Rivlin told Gantz "that in the current circumstances no extension would be possible."
"The president made his decision after also speaking to ... Netanyahu, who did not confirm in their conversation that the parties are close to signing an agreement that would lead to a unity government," a presidency statement said.
If no unity agreement is reached by the end of Monday, Rivlin will likely ask the Knesset to nominate a candidate to become prime minister.
Blue and White, a centrist alliance led by Gantz, said in a statement that "the attempts by the negotiating teams to arrive at a unity government are still ongoing."
Earlier Sunday, Netanyahu's right-wing Likud party said that it too remained open to forming a unity government.
Israel has confirmed nearly 11,000 cases of coronavirus and there have been widespread calls for an agreement on an interim government to led the country through the global pandemic.