NDJAMENA: Sudanese-backed rebels closed in on a key town in the east of the country Tuesday, said a Chadian minister, as the government denounced its neighbour for breaking a freshly signed peace deal.

The rebels swept deep into Chadian territory in hundreds of vehicles in a lightning offensive that began Monday, government spokesman Mahamat Hissene said.

Hissene, Chad's communications minister, said the rebels had penetrated around 100 kilometres (60 miles) inside Chadian territory and were closing in on the town of Goz Beida.

"They are on board several hundred vehicles. We've been tracking their movements since (they left) Sudan."

"However, for the moment there has been no contact with government forces," he added.

Inhabitants of the town reached by telephone told AFP all was quiet.

"The town is calm," said one young local, who asked not to be named.

"It is the aid agencies who let us know that the rebels had been spotted around Goz Beida," he added.

"There is no fighting here," said Ahmat, a driver. "There are no problems."

Earlier Tuesday, international sources had reported exchanges of fire between the Chadian air force and rebel columns heading for the town, but neither side has confirmed this.

"Our men are on a line between Goz Beida and Abeche," two key towns in eastern Chad, a rebel spokeswoman told AFP.

The arid central African country and neighbouring Sudan have for years traded accusations that each side backs rebels in the other.

Earlier, Hissene accused Sudan itself of sending "several armed columns" into Chad, also in violation of the peace agreement.

Khartoum has denied any involvement, but the Sudanese media centre, close to the intelligence services in Sudan, reported bloody clashes on Monday at the border between Chadian troops and rebels.

In February last year, rebels battled their way to the outskirts of the capital Ndjamena in a bid to overthrow President Idiss Deby Itno before being beaten back with logistical help from French forces based in the country.

France said Tuesday it was worried about the security of civilians.

"We are following with great concern the situation in the east of Chad," foreign ministry spokesman Eric Chevallier told a press conference in which he called on both countries to implement the peace deal.

The rebel assault is a blow to long-running diplomatic efforts to secure a thaw in relations between Khartoum and Ndjamena.

Brokered by Qatar and Libya, Sunday's deal was regarded as essential to any lasting settlement to the six-year-old uprising in the western Sudanese province of Darfur that has spilled over into Chad and the Central African Republic.

The UN mission in Chad (MINURCAT) said it has stepped up military patrols around Goz Beida and also restricted the movements of humanitarian personnel.

"For the moment, there is no major interruption to humanitarian activities, essential services remain active," said Katy Thiam, a spokeswoman for the UN Office for the Co-ordination of Humanitarian Affairs, from Abeche.

MINURCAT in March took over from a European force, EUFOR, responsible for protecting about 450,000 civilians in camps in Chad and in the north of CAR, where UN forces are also deployed.

France, the former colonial power in Chad, has 800 troops in the UN force, as well as a separate detachment of 1,500 troops at the local French regional base, which has in the past provided logistical support to the Chadian government when under attack.