Two major parties in Tunisia said on Friday they would not join the government of prime minister-designate Habib Jemli, meaning any coalition he does build may be fragile and increasing the risk of a political crisis.

The decisions by the Achaab and Attayar parties came after weeks of negotiations following October's election, which resulted in a deeply fractured parliament with no party winning more than a quarter of seats.

The decision will likely lead Jemli, who was put forward as prime minister by the moderate Islamist Ennahda party, to instead seek to bring the Heart of Tunisia party into his government.

Ennahda had earlier rejected sharing power with Heart of Tunisia, the party of media mogul Nabil Karoui, saying there were "suspicions of corruption against some of its leaders". Karoui faces corruption charges, which he denies.

Jemli this week told Reuters he expected to form a government next week and that political independents would hold most of the important portfolios, rather than members of major parties in the coalition.

"This way to form a government is not serious and does not show any identity to the next government. We are not interested in taking part in the Jemli government", the Achaab party said in a statement.

Mohamed Abbou, the leader of the Attayar party, said it would also not participate in the government or support it in a confidence vote after Jemli refused to give the party the ministries of interior and justice.

Tunisia needs a strong government to face a string of challenges, including protests by frustrated young people demanding jobs and development, and pressure from international lenders demanding fiscal reforms.

However, even if Jemli can form a coalition mustering enough parliamentary support to survive a vote of confidence, the small number of parties involved would likely make it fragile and vulnerable to political pressure.

Analysts say a weak government that does not enjoy much political support may prove unable to better the record of past governments in resolving the economic problems that have plagued Tunisia since its 2011 revolution.

The Tahya Tounes party, led by the current Prime Minister, Youssef Chahed, has said its natural place was the opposition.

Tunisia's economy has suffered years of low growth since the uprising that ended autocracy and introduced democratic rule, with successive governments struggling to create jobs and tame inflation.