Three Israeli parties on Saturday proposed forming a centre-left opposition bloc to try to topple conservative Prime Minister Benjamin Netanyahu in the Jan. 22 election, but disagreements over the terms suggested any pact could be elusive.

Former foreign minister Tzipi Livni, head of the centrist Hatenuah party, said on Twitter that she and leaders of the centrist Yesh Atid and left-leaning Labour parties would "discuss the creation of a 'united front' to work together to replace Netanyahu".

Opinion polls suggest the three parties could collectively win about 37 of parliament's 120 seats, more than the some 35 seats projected for Netanyahu's rightist list and potentially enough to form the next Israeli coalition government.

Livni's proposed partners agreed to meet her in the coming days but quickly disagreed over whether the centre-left should remain open to the idea of being part of the ruling coalition.

A unity government with Netanyahu has been ruled out by Labour, whose leader, Shelly Yachimovich, said this week she intended either to be the next prime minister or to sit in opposition.

By shunning any future partnership with Netanyahu, challengers could "plant enormous hope in the heart of the public ... and bring about grassroots mobilisation for a determined and spirited struggle," Yachimovich said in a statement on Saturday.

"A unified move by ... all those who seek to change the government will be real and meaningful only if such parties act as we did," Yachimovich said.

Describing Netanyahu's re-election as a foregone conclusion, Yesh Atid leader Yair Lapid said he wanted the centre-left to be open to the idea of joining the coalition so as to offset the premier's religious-nationalist allies.

"Let's create a taking-the-initiative bloc, to create a unity government," Lapid said on Channel Two television.

Livni has cast herself as an alternative to Netanyahu but stopped short of saying she would not join him in government.

Netanyahu, in his second term as premier, takes credit for the relative stability of the Israeli economy and appeals to the Jewish state's burgeoning religious-nationalist sectors by championing the settlement of occupied land. He has taken a hawkish line on the Palestinians and Iran but avoided major wars.

Israel's festering international isolation has been seized on by Livni, who as top diplomat in the former government pursued inconclusive talks on founding a Palestinian state.

Speaking separately to Channel Two this week, Netanyahu said that were Livni to serve under him in the next government, she would not lead peacemaking efforts.

Lapid and Yachimovich are new to politics and known to much of the public from their previous careers as television commentators. Their campaigns have focused on social reform.

The Netanyahu government was unfazed by Livni's outreach to fellow challengers.

"I wish that the other side, to the left, would coalesce, because that would hone the differences between us," Vice Prime Minister Moshe Yaalon of the ruling Likud said in a speech.

In an apparent dig at Lapid and Yachimovich, Yaalon rued "the immodesty and immaturity in the desire of certain people to jump straight into the cold water of being prime minister, without passing through any stations along the way".