The latest fighting in east Chad has disrupted humanitarian operations and raised the risks for a European peacekeeping force due to deploy there soon, aid officials and diplomats said.

In the biggest clash for months between Chad's government forces and eastern rebels, the army said on Monday it had killed hundreds of insurgents near the border with Sudan's Darfur region.

Commanders of the rebel Union of Forces for Democracy and Development (UFDD) put army losses at 200 killed and said they expected further attacks by President Idriss Deby's forces.

There was no independent confirmation of the casualties.

Reporters who visited the site of the fighting at Abou Goulem, some 60 km from the Darfur border, said they saw wrecked vehicles, including five armoured assault cars, strewn across a battleground that stretched over a wide area.

Two Chadian rebel groups ended a ceasefire at the weekend to break a fragile calm following the Oct 25 signing of a Libyan-brokered peace deal between Deby and his main rebel foes.

Chad's foreign minister summoned Sudan's ambassador to protest against the attack, which Chad says was launched from Sudan.

Chad and Sudan have often accused each other of supporting rival armed rebel factions, particularly during the past two years of rebellion in Chad's east against Deby's 17-year rule.

Foreign diplomats said the Sirte peace deal had appeared shaky from the outset and the latest violence highlighted the dangers for up to 3,700 European Union peacekeepers who are due to start arriving in eastern Chad early in the New Year.

'This is a forewarning ... nobody from the EU is going to feel confident now that there has been heavy fighting,' one Chad-based diplomat said.

Deployment of the EU force for eastern Chad, initially scheduled to start in November, has been delayed.

Its backers are trying to find a full complement of troops among EU members and are seeking commitments of aircraft and medical support units.

Relief workers in the region have been clamouring for more than a year for international protection for more than 400,000 Sudanese and Chadian civilian refugees who have been forced from their homes because of violence in Darfur and in eastern Chad.

'We'd like the protection right now, but we will have to live with (the situation) until they come, and hopefully by then things will be calmer,' Annette Rehrl, spokeswoman in Chad for the UN refugee agency UNHCR, said in Abeche.

She said the fighting over the last few days had bottled up  more than 100 aid workers in the area around Forchana, where UNHCR runs two refugee camps not far from the Sudanese border.

UFDD secretary general Abakar Tollimi said rebel forces were still near Am-Zoer, northeast of Abeche, the hub of foreign humanitarian operations in the east of the landlocked country.

'They (the government) will definitely try to attack us again ... but we have given them a lesson,' he told Reuters.

Rehrl said the UNHCR could evacuate some workers from the region if the security situation deteriorated further.

'In eastern Chad things can change every hour -- an evacuation is a very big operation so we hope that we won't be forced to evacuate and that things will calm down,' she said.

In a statement, the Chadian army said it destroyed 40 rebel vehicles and seized 50 more, killing several hundred insurgents and taking prisoners.

The UFDD and a second rebel group, the Assembly of Forces for Change (RFC), said on Friday they would break a ceasefire after the government failed on commitments to mutually agree mechanisms to integrate rebels into the army. Reuters