An Ethiopian court acquitted two Swedish journalists on Thursday of conspiring to commit terrorist attacks, but kept them in detention and said they must still answer allegations they had assisted the secessionist ONLF rebel group.

Reporter Martin Schibbye and photographer Johan Persson were arrested in July after they entered Ethiopia's Ogaden province from Somalia's semi-autonomous Puntland region with a team of fighters from the Ogaden National Liberation Front (ONLF).

The freelance journalists say they were covering the ONLF as a news story and deny assisting their cause.

Though the prosecutors have provided witnesses and evidence to support their accusation, the court does not believe it is substantial enough to prove that the journalists were involved in carrying out a terror attack, judge Shemsu Sirgaga ruled.

Shemsu, however, ordered the pair to defend themselves against allegations they had helped the ONLF and had entered the country illegally without a permit.

Persson and Schibbye last month pleaded not guilty to the terrorism charges but admitted to crossing the border without a permit.

A security official and soldiers who appeared in court as prosecution witnesses have said the pair were tracked down with a group of armed rebels some 90 kilometres (56 miles) inside Ethiopia.

The government launched a security operation against the militants, killing 20 fighters and seizing the two journalists.

Two rebels caught alongside the Swedes were found guilty of attempting to carry out attacks.

During the trial, prosecutors screened a three-hour video they said was obtained from one of the Swedes' laptops, showing footage of the journalists hoisting rifles alongside armed men and being briefed over a map on how to infiltrate the region.

The clip also included interviews with unidentified individuals who described abuses in the region.

The journalists' lawyers told the court that they would challenge the two outstanding charges, saying they would provide up to eight witnesses to prove their innocence. The defendants will next appear in court on December 6.

Addis Ababa has blacklisted the ONLF as a terrorist group, and its recently-adopted anti-terrorism legislation outlaws promotion of the insurgents' activities.