Tunisia's ruling Islamists on Monday rejected a plan under which they would step down pending elections, a decision likely to deepen confrontation with secular opponents demanding their immediate resignation.

Tunisia, whose 2011 uprising was the first of a series across the Arab world, has been in turmoil since an opposition leader was assassinated in July, threatening a democratic transition once seen as the most promising in a troubled region.

The country's powerful UGTT union had been pushing both sides to accept a plan for the Islamist-led government to step down after three weeks of talks to decide on a date for elections and the composition of a new caretaker administration.

But the moderate Islamist Ennahda party called on Monday for more guarantees on the election date and said an assembly writing a new constitution should finish its work before the government agreed to relinquish power.

"We have said that this government would not step down concretely before the completion of the constitution," Rafik Abd Essalem, a senior Ennahda official, told reporters.

Since former autocrat Zine al-Abidine Ben Ali fell in 2011, Tunisia has faced growing divisions over the political role of Islam, with the opposition accusing Ennahda of promoting an Islamist agenda in one of the Arab world's most secular nations.

After suspected Islamist militants shot dead an opposition leader in July, the second such assassination this year, Ennahda came under mounting pressure and street protests from an opposition emboldened by events in Egypt, where the military deposed Islamist president Mohamed Morsi following a nationwide uprising the same month.

This story was edited by Ahram Online.