Chad's government has deployed troops outside the capital, N'Djamena, in case of a rebel attack, but it denies that the city is under threat.
Pick-up trucks packed the soldiers are patrolling the capital
Rebels say its advance on N'Djamena has stopped, but its war of attrition against President Idriss Deby goes on.
Rebel leader Mahamat Nouri has told the BBC that his rebel allies are just over 200km north of the capital.
But a BBC reporter in N'Djamena says the city is tense and hundreds of armed soldiers are patrolling the streets.
Earlier, the government said it had retaken two towns near the Sudanese border, a day after rebels seized them, but the rebels said they were withdrawing from the towns.
In April, a rebel column advanced from the Sudanese border to the outskirts of N'Djamena before being repelled.
Abeche, about 700km (440 miles) east of N'Djamena, and the nearby town of Biltine, were captured by two different groups opposed to the rule of President Idriss Deby on Saturday.
The BBC's Stephanie Hancock in the capital says the government may be saying that there is no threat of a rebel attack, but the reaction of the Chadian army suggests otherwise.
Pick-up trucks packed with soldiers are patrolling the town and two tanks are parked near the presidential palace.
Hundreds of soldiers can be seen walking about the streets, some of them heavily armed, she says.
An administrative building near the presidential palace, which is normally home to five government ministries, has been completely emptied of staff and dozens of soldiers armed with rocket-propelled grenades and other arms are now stationed outside.
The exact position of the rebels said to be threatening N'Djamena is not known.
In a statement sent to the AFP news agency, the French embassy said: "The military situation changed swiftly at the end of the morning. The presence of a large rebel column has been confirmed in the Bata region of the country, heading west."
That sighting put the rebels at less than 400km from the capital.
However, the embassy later softened its advice to say the column "was no longer progressing" and that "the situation is normal in N'Djamena".
But Mr Nouri, leader of the Forces for Development and Democracy (UFDD) rebel group which attacked Abeche at the weekend, told the BBC that the rebels who are not from his group are north-east of the town of Moussouro, just over 200km from the capital.
Chad's Defence Minister Bichara Issa Djadallah dismissed this as lies and said all the rebels are currently in the region of Biltine, more than 600km away on the other side of the country.
Nonetheless, he said government forces were preparing to deal with a rebel offensive, with army troops mobilised and massing outside N'Djamena.
He said UFDD had fled Abeche on Sunday morning after the Chadian Army surrounded their positions.
Extensive looting took place in Abeche on Saturday night, and there are reports that at least three humanitarian compounds were targeted.
Abeche, about 160km from the Sudanese border, is the centre of a huge relief operation, to assist refugees from the conflict in Sudan's Darfur region.
About 200,000 refugees have crossed the border into Chad to escape the violence in Darfur.
The Chadian government has accused Sudan of backing the rebels - a charge Sudan denies - while Chad denies Sudanese claims that it supports black African rebels in Darfur.