The United States said rebel fighters in Chad appeared to be moving towards the capital N'Djamena and ordered non-essential staff to leave, warning of possible violence.

A spokesperson for the rebel Front for Change and Concord in Chad (FACT) said its fighters had "liberated" the province of Kanem, some 220 km from the capital N'Djamena, but the government denied this.

"The authors of these false statements are not even on the ground, but somewhere in Europe," the government said in a message posted to Facebook.

A day earlier the British government urged its citizens to leave Chad because of information that two rebel convoys on the move, one near the town of Faya, some 770 km (478 miles) northeast of N'Djamena, and another by the town of Mao, the provincial capital of Kanem.

On Sunday morning, a Reuters reporter saw large numbers of heavily armed Chad security forces patrolling the streets of the capital.

Partial election results showed President Idriss Deby on course to extend his three decades in power, despite signs of growing discontent over his handling of the nation's oil wealth.

Deby has won a majority in all but two of the 84 departments announced so far, and secured a plurality in the other two, with 28 departments remaining, according to the Independent National Election Commission (CENI).

Deby, who seized power in 1990 at the head of an armed rebellion, is a staunch ally of France and the United States in the fight against Islamist militants in the arid Sahel region.

"Due to their growing proximity to N'Djamena and the possibility for violence in the city, non-essential US government employees have been ordered to leave Chad by commercial airline," the US State Department said in a statement late on Saturday.

Chad's army said it had destroyed a rebel convoy in the north of Kanem province on Saturday afternoon.

"The column was totally decimated," army spokesperson Azim Bermandoa Agouna said in a statement late on Saturday.

FACT, which is based on Chad's northern frontier in Libya, attacked a Chadian border post on the evening of April 11, just as polling stations were closing.

FACT spokesperson Kingabe Ogouzeimi de Tapol said rebels had "liberated" Kanem and were still trying to root out remaining state security forces.

"We reassure the population of the city of N'Djamena and its surrounding area, in particular diplomatic personnel, United Nations staff, partners, and expatriates working in Chad, to remain calm and to avoid any non-essential travel outside the city," Ogouzeimi said in a statement posted to Facebook.

A group of 14 opposition leaders, who had called for their supporters to boycott the election, signed a petition on Sunday calling for a ceasefire to allow for an "inclusive national dialogue."