As many as 45 international observers who were part of the UN peacekeeping mission in Syria are to be tried according to "divine [Islamic] Sharia law", a frontline al-Qaida group in the war-torn country has announced.

The al-Nusra Front, the active al-Qaida branch in Syria, has said it will put the Fijians, who were captured last week in Golan Heights, on trial under Islamic law as there has been no breakthrough in talks with Fijian authorities, according to reports.

Al-Nusra said the Fijian troops were captured by it as the UN continues to ignore "the daily shedding of Muslims' blood in Syria" at the hands of Syrian President Bashar al-Assad.

Earlier, the group stated three key demands to release the observers: removal of the group from the UN terror list; delivery of humanitarian aid in Damascus; and compensation for the death of some rebel fighters.

The UN Security Council had condemned the capture "in the strongest terms" and demanded the "immediate and unconditional release" of the observers.

"We are sparing no effort to obtain the release of the detained peacekeepers," UN peacekeeping chief Herve Ladsous told reporters.

Although several countries have withdrawn their troops from the UN peacekeeping mission in Syria, six countries - Fiji, the Philippines, the Netherlands, Nepal, India and Ireland - have maintained their forces.