Sudan's government is arming rebels in Chad, the government has alleged amid reports that rebels are moving towards the Chadian capital, N'Djamena.
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 are now 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 reports of the rebels' location are changing almost hour by hour.
"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.
BBC World Service broadcasts to Juba in Southern Sudan and surrounding areas in Arabic on 90.0 FM and in English on 88.2 FM. In Northern Sudan: Khartoum 91.0 FM, Al Ubayyid 91.0 FM, Port Sudan 91.0 FM and Wad Madani 91.5 FM.
“Violence will bring no victory. For the sake of the Darfur civilians, all parties to the conflict have to stop resorting to violence. This is the absolute priority.” - Luis Moreno-Ocampo, Prosecutor of the International Criminal Court, 05 June 2009
"APPREHENSION" by Rob Rooker. Painted on a wall in Maridi, Sudan. The image is of a young Nuer boy looking up among a crowd of people. Click on image for more details. Cards & prints of Rob's paintings can be purchased online at Imagekind.com