N'DJAMENA, Chad (Reuters) -- Chad said Friday that a "state of belligerence" existed between itself and Sudan following attacks on an eastern border town by rebels it said were armed and directed by the Sudanese government.

While President Idriss Deby's government stopped short of declaring war or breaking ties with Sudan, the statement was the toughest so far against its eastern neighbor over the December 18 attacks against the border town of Adre.

"The government calls on the Chadian people to mobilize against this Sudanese aggression," the government said.

"Chad is today in a state of belligerence with Sudan," it added, describing Sudanese President Omar Hassan al-Bashir as an "enemy of Chad."

The declaration reflected sharply increased tension over the volatile border area, which includes Sudan's western Darfur region where tens of thousands of people have been killed in fighting between Sudanese government forces and militias and Sudanese rebels in the last few years.

Chad's Foreign Minister Ahmat Allam-Mi said the Sudanese ambassador in the capital N'Djamena had been summoned to the ministry Friday and given a memorandum listing "Sudan's aggressions against Chad."

"We demanded that Sudan cease all aggression against our country, and we told him that Chad has no bellicose feelings against Sudan," the minister said.

Chad has accused the Sudanese government of directing Sunday's twin assaults against Adre by Chadian rebels of the recently formed Rally for Democracy and Liberty group, which has vowed to topple Deby.

The Chad rebels include army deserters and former comrades in arms of Deby who helped bring him to power in a 1990 rebellion. But they now say he is corrupt and vow to remove him from the leadership of Africa's newest oil producer.

Chad says its forces repulsed the assaults, killing about 300 of the attackers, pursuing the rebels over the border into Sudan and destroying their bases there.

The Rally for Democracy and Liberty rebels reject this government version of the battle for Adre. They say they lost nine men and that more than 70 government troops were killed. They also say they destroyed two helicopters and several tanks and other vehicles.

There has been no independent confirmation of either version.

"The Sudanese government ... took the initiative to attack Chad on December 18 2005, by arming, equipping and forming the columns that assaulted Adre aboard brand-new Toyota trucks carrying heavy weapons provided days earlier by the Sudanese defense minister," the Chadian government statement said.

Local and foreign journalists taken to Adre this week by Chad's government say they were shown destroyed pickups and what officials identified as the bodies of slain rebels lying in the sand and dry scrub of the battlefield.

To rally Adre's inhabitants and troops, Deby visited the town Thursday and dismissed threats to overthrow him.

Chad urged the U.N. Security Council on Thursday to help the stop conflict in Darfur and the border area from spreading.

It said Friday it welcomed condemnation of the attacks on Adre by the United Nations and by foreign governments such as France and urged the African Union and other regional bodies to do the same.

Following the Adre attacks, Chad has made clear it objects to Sudan hosting a scheduled summit of AU heads of state in Khartoum next month.

It also has called on the AU not to allow al-Bashir to become the next chairman of the continental body.