Tunisia's main parties rejected a proposal on Thursday by the moderate Islamist Ennahda party to name a prime minister from its own ranks after its win in last month's parliamentary elections.
Deadlock in the fragmented Parliament would complicate Tunisia's efforts to address chronic economic problems, including a large public debt and 15 per cent unemployment.
Ennahda will be the largest party in Tunisia's new Parliament but with only 52 of 217 seats, forcing it to compromise to form a Cabinet.
It said it had decided that one of its leaders should be prime minister because Tunisians had given it the responsibility to implement its electoral programmes.
But the proposal was not accepted by potential coalition partners Attayar, which has 22 seats, Achaab's Movement with 16 seats and Prime Minister Youssef Chahed's Tahya Tounes, with 14.
Only the conservative Karama coalition, with 21 seats, agreed.
In a second meeting with Ennahda on Thursday, Attayar said it would not accept a prime minister from that party, and asked for the portfolios of justice, interior and administrative reform in the new government.
Gazi Chaouachi, a leader in Attayar, said Tunisia needed a consensus prime minister from outside Ennahda, with an economic background to help save the ailing economy.
Thaya Tounes also said it would not join a Cabinet led by Ennahda and called for a government of "national interest" focused on urgent economic reforms.
Ennahda has excluded two parties from talks. They are Heart of Tunisia, which holds 38 seats and is led by the media magnate Nabil Karoui, and the Free Constitutional Party led by Abir Moussa, with 17 seats, which opposes Ennahda.
The moderate secularists are expected to reveal their choice for prime minister by the end of this week or early next.
Possible choices include party leader Rached Ghannouchi and Zied Ladhari, the minister of investment who resigned on Thursday.
If Ennahda cannot form a government within two months, the president can ask another group to try.
If that fails and the deadlock persists, there will be another election.
