By Tarek Amara

TUNIS (Reuters) - Tunisia's ruling Islamists have failed to bring more secular parties into a coalition government due to be formed by Friday to oversee elections in a transition process jolted by the assassination of a leftist politician.

Two secular parties, Democratic Alliance and Wafa, said on Thursday they had decided to stay out of the next government to be led by the Islamist Ennahda party.

Under the constitution, Ennahda's Ali Larayedh has until Friday to announce his ministerial line-up, 15 days after he was assigned by President Moncef Marzouki to form a government.

Larayedh had held consultations with five parties, but only Marzouki's secular Congress for the Republic (CPR) has agreed to join his coalition, while the center-left Ettakatol party is still negotiating.

The same parties served in the last government led by Hamadi Jebali, who quit after his own Ennahda party rejected his plan for a technocrat cabinet to organize elections and calm unrest after the February 6 killing of secular politician Chokri Belaid.

Larayedh was meeting representatives of the three parties on Thursday to seek agreement on who should head the interior and justice ministries. Political sources said Ettakatol had rejected the prime minister-designate's nominees for the posts.

"There is a large degree of agreement on the next government and we will continue today to discuss controversial subjects," Larayedh said.

The political sources said Abdelhak Lassoued, an independent, would be defense minister, career diplomat Othman Jarandi would take over as foreign minister, while Ettakatol's Elyess Fakhfakh would keep his job as finance minister.

Democratic Alliance, a small secular party, said it had quit the coalition talks because it felt Ennahda was not really ready to cede control of the interior and justice ministries.

"We opposed the proposals of Ennahda to appoint people close to them in the ministries of interior and justice," said Mahmoud Baroudi, a leader in Democratic Alliance.

Wafa also said it would stay out of the new government, which it said would follow the same policies as the last one.

Ennahda's leader Rached Ghannouchi had told Reuters earlier that five parties would take part in the next government, saying a coalition of moderate Islamist and secular groups was vital.

Ennahda, Ettakatol and CPR have ruled in coalition since December 2011, following Tunisia's first free election. A popular uprising had overthrown autocrat Zine al-Abidine Ben Ali in January 2011, inspiring a wave of Arab revolts elsewhere.

Larayedh is expected to announce his cabinet later on Thursday or on Friday. No date for elections has been set and work on a new constitution has yet to be completed.

Political turmoil in recent weeks has set back talks with the International Monetary Fund on a $1.78 billion loan and has prompted Standard and Poor's lower its long-term foreign and local currency sovereign credit rating of Tunisia.

(Reporting By Tarek Amara; Editing by Alistair Lyon)