Sudan's government is arming rebels in Chad, the government has alleged after security was tightened in N'Djamena following rebel attacks.
The army has been unable to end the rebellion
Chad's foreign minister said the proof was the firing of a missile at a French reconnaissance plane in the east.
The rebels began their offensive in the east at the weekend but were then said to be near the central town of Mongo.
A BBC correspondent in N'Djamena says tanks are stationed in key areas, such as outside the presidential palace.
Troops have been recalled to base despite the Muslim holiday of Eid - the biggest festival of the year in Chad.
The BBC's Stephanie Hancock in Chad says the outskirts of the capital are said to have been heavily fortified with government troops.
But she says the latest reports of the rebels' location suggest they may have pulled back.
"These rebels entered Chad from Sudan and they could only have procured this type of military equipment within the sight of and with the knowledge of the Sudanese authorities. Sudan cannot deny it," Chadian Foreign Minister Ahmar Allami told AFP news agency.
Khartoum denies backing the rebels, and in turn accuses Chad of backing rebels in the war-torn Darfur region.
Eastern Chad has a similar ethnic make-up to Darfur, where Arab militias are accused of carrying out a genocide against black Africans.
On Monday night, the rebels claimed to have seized the town of Am Timan, some 600km from N'Djamena but the government denied it had fallen and urged the capital's residents to stay calm.
"The government appeals to the population to remain calm and to go normally about one's business," said spokesman Hourmadji Moussa Doumgor.
On Sunday, the Union of Forces for Democracy and Development (UFDD) rebel seized the small town of Gos Beida.
Our correspondent says there was also fierce fighting in the border town of Ade.
Our reporter says the role of the French army here will also be key to how the struggle for power plays out.
Back in April, when rebels managed to reach the heart of the capital in a four-day advance before being overpowered, the French received widespread criticism from Chadians for the way they staunchly supported President Idriss Deby.
The French army has also admitted that two days ago one of their surveillance planes narrowly avoided being shot down by a surface-to-air missile fired by rebels.
While the French do still openly support the Chadian government, there are some questions over exactly how far they will go to prop up President Deby's regime - and how much they are willing to risk, our reporter says.