Yemen's armed tribesmen are demanding President Ali Abdullah Saleh to leave power as agreed on Saturday to end the conflict that brought Arabian country to the brink of civil war.

The deal includes withdrawal of armed tribesmen from government buildings and to stop the week-long clash that killed 115 people, a source close to mediators told Reuters.

Yes, we have an agreement which takes effect tomorrow (Sunday) morning, a government official told Reuters.

The clashes have driven thousands of residents to flee Sanaa and this has raised the panorama of chaos which could help the Yemen-based subdivision of al Qaeda and threaten the neighboring Saudi Arabia, the world's biggest oil exporter.

Latest violence, between Saleh loyalist forces against members of the powerful Hashed tribe, resulted in bloodiest since the pro-democracy protests erupted in January and was sparked by Saleh's refusal to sign the power transfer deal.

Peacekeeping troops started taking over the occupied buildings from the al-Ahmar tribesmen on Sunday and handed them over to government forces.

In another development in southern Yemen, three French aid workers went missing and a local security official said they were believed to have been abducted.

Kidnappings of Western tourists or workers by disgruntled tribes seeking ransom or concessions from the government have been frequent in Yemen. Most hostages have been freed unharmed.

The International Crisis Group (ICG) said a broad and lasting ceasefire is needed immediately besides a plan that leads to the change of power.

To prevent further escalation and loss of life, the most urgent step is for both sides to immediately accept a ceasefire mediated by Yemen's statesmen and tribal leaders, the ICG said in a conflict risk alert issued late on Friday.

It said foreign states should be involved, but, given the deeply personal and tribal nature of the feud between the Salehs and al-Ahmars, it cannot be addressed effectively by international mediation or initiatives alone.

On Friday, Yemeni tribesmen said they had captured a military compound from elite troops loyal to the president about 60 miles (100 km) outside Sanaa.

The fighting has overshadowed a largely peaceful protest movement that started months ago aimed at ending Saleh's 33-year-long tyrannical rule which was inspired by the uprisings that brought down the long-standing leaders of Tunisia and Egypt.