The hardline opposition Likud party Tuesday rejected an invitation by Israeli Foreign Minister Tzipi Livni to join a government of national unity led by her centrist Kadima faction.

"Livni is trying to act in a stately manner, but she's coming across as pathetic," a confidant of Likud leader Benjamin Netanyahu told the Israeli y-net news site.

The 12-mandate Likud earlier overnight issued a statement, formally rejecting the offer and calling for early elections.

"The citizens of Israel must be allowed to choose who will lead them and how," it said.

Israeli President Shimon Peres Monday evening formally charged Livni with forming a new government, a day after Ehud Olmert resigned from the premiership and less than a week after Livni won the leadership of Israel's ruling Kadima party in an internal primary.

The 50-year-old now has up to 42 days to form the new coalition. If she fails, new elections will be held within 90 days, or by March 2009, a year early.

In a news conference at the president's residence in Jerusalem late Monday, she said that her priority was political "stability," but that she would not hesitate to go for early elections if this proved impossible.

Kadima, with 29 mandates, is the largest party in Israel's 120-seat parliament. Confidants of Labour Party leader Ehud Barak have said chances have grown that it will join the coalition. With 19 mandates, Labour is currently Olmert's largest coalition partner.

Livni is also negotiating with the ultra-Orthodox, 12-mandate Shas party, which is demanding higher child allowances and a guarantee that she will not negotiate the status of Jerusalem with the Palestinians.

The Pensioners' Party, which has 7 mandates, has already said it will also stay in the new coalition.