Sharp fighting broke out between rebel and government forces in northern Chad today, but Paris denied rebel claims that French warplanes bombed rebel positions near the isolated government outpost of Oum Chalouba, 400 miles northeast of the capital of Ndjamena.

The Chad government of President Hissene Habre today claimed its troops killed 800 Libyan-backed rebels and captured 600 after repulsing an attack on the desert garrison of Oum Chalouba, according to news agency accounts from Ndjamena.

An Army communique said most of the men captured after a battle were Sudanese. It said the figure for men killed was provisional and gave no further details of the fighting. The communique also said government forces captured 12 vehicles and anti-aircraft and ground-to-ground missiles mounted on them and 16 large Mercedes trucks loaded with ammunition and food.

Government forces are still reported to be in control, Chadian and French officials said.

More than 3,000 Libyan soldiers and Chadian rebels launched a dawn attack on Oum Chalouba, government officials said. Chadian Information Minister Mahamat Soumaila said the force, backed by tanks and artillery, attacked the town of Oum Chalouba twice but was successfully repulsed.

Speaking to a reporter at a dinner to commemorate the 200th anniversary of the Treaty of Paris, French Foreign Minister Claude Cheysson said that according to French information, the fighting in Oum Chalouba had not been nearly as serious as had been reported by the government in Ndjamena. He said that both the Habre government and the Libyan-backed rebels appeared to be exaggerating their accounts of their fighting, and he explicitly denied reports that French warplanes had taken part.

The attack on Oum Chalouba broke the three-week-old tacit cease-fire between the Libyan-backed rebel forces led by former president Goukouni Oueddei and the Chadian government of Habre. The rebel forces, along with an estimated 3,500 Libyan soldiers, have taken over the northern third of Chad since June.

In a statement denying claims that French Jaguar jets fired on rebel forces--the first report of French aircraft being used in the Chad fighting--French Defense Minister Charles Hernu said, "The French Jaguars in Chad today carried out a training and reconnaissance mission without firing, as they do every day."

The official mission of the four Jaguar combat jets and the four F1 Mirages, which have been stationed in Ndjamena since Aug. 21, is to assure the protection of the more than 3,000 French troops sent to Chad as instructors to back Habre.

French President Francois Mitterrand has said the French troops here will not restrict themselves to being a defensive bulwark for Habre's government and might retaliate against Libyan forces if fired on.

French Defense Ministry sources have acknowledged the planes flew over the French defense line that connects French units in Salal in the west and Abeche, Arada and Biltine in the east. It is unclear, however, whether they could also have flown over Oum Chalouba, which lies outside the French zone about 95 miles north of the closest French Army positions in Arada.

Abderrahman Moussa, a former ambassador for Goukouni in Paris, said rebel positions in Oum Chalouba were bombed by French Jaguars during "a provocation attack" by Habre's forces. The report came after Chadian Information Minister Soumaila told reporters in Ndjamena that rebel forces attacked the government outpost.

Oum Chalouba is on one of the two roads leading south from rebel-held Faya Largeau in the north.

Western military sources said last month that the Chadian strongpoint there was militarily indefensible if the Libyans ever launched a major attack against it.

Chadian Information Minister Soumaila said the French troops have not yet been involved in any fighting and said there was no immediate prospect that they would be since the Libyan-backed forces were on the retreat.

French intervention in August halted the Libyan and rebel advance south of Faya Largeau. Libya has fortified the desert town with Soviet-made tanks and armored cars and has moved in the gasoline that will be needed if the rebels try to drive south.