JERUSALEM: Prime Minister Benjamin Netanyahu and his choice for defence minister, ultranationalist Avigdor Lieberman, signed a coalition agreement on Wednesday and issued assurances that the most right wing government in Israel's history would act responsibly.

Once Lieberman is sworn in, Netanyahu will have a government of 66 legislators, widening his current one-seat majority in the 120-member parliament, a goal the Israeli leader has said he has sought since winning a fourth term last year.

Lieberman's return to office - he was previously foreign minister - has raised questions at home and abroad given his past criticism of Israel's Arab minority, U.S.-sponsored peace talks with Palestinians and regional powers Egypt and Turkey.

At the signing ceremony, in which Lieberman's Yisrael Beitenu party formally agreed to join Netanyahu's Likud, both men switched from Hebrew to English to deliver a message to the international community.

"My government remains committed to pursuing peace with the Palestinians, pursuing peace with all our neighbours," said Netanyahu. "My policy has not changed. We will continue to pursue every avenue for peace, while ensuring the safety and security of our citizens."

He said a broader and more stable government would make it easier to "seize new opportunities" in the region, a reference to potential peace moves with Arab states that share Israel's concern about Islamist militancy and Iran.

Palestinian officials said that with Lieberman, who lives in a settlement in the occupied West Bank, back in the cabinet as defence minister prospects for reviving statehood negotiations that collapsed in 2014 had grown dimmer.

But also speaking in English, Lieberman, who once famously threatened to bomb Egypt's Aswan dam and has called for the assassination of Hamas Islamist leaders in Gaza, promised a "responsible and reasonable" policy.

"At the end of the day my intention (is) to provide security and of course all of us we have a commitment, strong commitment, to the peace, to the final status agreement (with the Palestinians)," said the Soviet-born party leader.

Nabil Abu Rdainah, the spokesman for Palestinian President Mahmoud Abbas, said: "What's important is deeds not words... "Israel should learn the true lesson from making peace because there can be no peace and no stability in the region unless the Palestinian cause is resolved."

Netanyahu began negotiations with Lieberman last week after coalition talks failed with the centre-left Zionist Union, the main opposition. The courting of Lieberman came as a surprise as he and Netanyahu have been sharply dismissive of one another.

Several former Israeli defence ministers have criticised Lieberman's appointment to the sensitive post, citing the politician's relative lack of military experience.

Yisrael Beitenu has six legislators, but one of them, Orly Levi-Abekasis, has said she is leaving the party and would vote independently in parliament, citing what she called its failure to pursue economic and social reforms.

Levi-Abekasis's spokesman said on Wednesday she was still formally a member of Yisrael Beitenu as procedural issues had yet to be finalised.

Yisrael Beitenu will become the sixth party in Netanyahu's religious-nationalist coalition. The deal prompted Moshe Yaalon, a Likud member and former general, to quit as defence minister in protest on Friday.

He could emerge as a future challenger to Netanyahu.