Tunisians on Thursday mourned six policemen killed in a firefight with suspected jihadists, as long-awaited crisis talks faced fresh delay over opposition doubts about the ruling Islamists' readiness to quit.

The slain police were to be buried later Thursday in home towns around Tunisia, including in the central Sidi Bouzid region, where Wednesday's clash broke out, amid rising anti-government sentiment.

In Kef, where one of the officers is to be buried, protesters torched the office of the ruling Islamist party Ennahda on Thursday morning.

President Moncef Marzouki has declared three days of national mourning for the officers, but the victims' families have barred government representatives from attending the funerals, and no official ceremony is planned.

The powerful UGTT trade union confederation called a strike in the poor central region where the uprising that toppled strongman Zine El Abidine Ben Ali in 2011 first began, and in nearby Kasserine.

Wednesday's violence came a week after two policemen and nine "terrorists" were killed in Beja, west of the capital, and as opposition protesters massed in central Tunis demanding the immediate resignation of the Islamist-led coalition.

Some demonstrators have vowed to continue their protest everyday outside the Kasbah, the site of the government's headquarters, where dozens remained camped out overnight.

A national dialogue, which is the centerpiece of a plan to end the political paralysis gripping the country since the July assassination of opposition Parliament member Mohamed Brahmi, has been put back to Friday.

The roadmap, drawn up by mediators led by the UGTT, ran into trouble when the Islamist premier gave what the opposition described as an "ambiguous" commitment to step down in a speech late on Wednesday.

The UGTT chief said the premier needed to clarify his comments to get the dialogue back on track.

According to the roadmap, the talks must lead within three weeks to the formation of a caretaker cabinet of technocrats.

Negotiators will also have one month to adopt a new constitution, electoral laws and a timetable for fresh elections - key milestones in a transition that has effectively been blocked by wrangling between the Islamists, their coalition allies and the opposition.

Commenting on the latest violence, Prime Minister Ali Larayedh insisted Tunisia was "in the process of defeating terrorism... despite the sacrifices," and that the security forces were pursuing the remaining militants.

Marzouki said Tunisia's high security council would meet on Thursday to discuss the jihadist threat, but gave no further details.

The defense ministry has admitted it lacks the resources to combat militant groups and has struggled to contain them.

AFP