The Chadian government has accused Sudan of launching a military assault, two days after the two countries signed a reconciliation agreement in Doha, the Qatari capital.
"While the ink has yet to dry on the Doha accord, the Khartoum regime has just launched several armoured columns against our country," Mahamat Hissein, a Chadian government spokesman, said on Tuesday.
Hissein accused Khartoum of a "planned aggression", but did not say whether Sudan's forces had entered Chad's territory or if they had stopped at the border.
A Sudanese army spokesman denied the accusations.
"What is happening now inside Chad is between the Chadian army and the Chadian fighters," Osman al-Agbash, the Sudanese army spokesman, said.
"Sudan has no relation with this".
After the talks in Doha on Sunday, Sudan and Chad struck a deal to end hostilities and arrange a summit between their leaders in a move seen as vital for peace-making efforts in Darfur in western Sudan.
Diplomats have been keen to secure a thaw in relations between Khartoum and Ndjamena, the Chadian capital.
They regard it as essential to any lasting settlement to the six-year-old uprising in Darfur that has spilled over into Chad and the Central African Republic.
Qatar and Libya have been leading reconciliation efforts between Chad and Sudan after they restored diplomatic ties in November.
Chad and Sudan accuse each other of arming anti-government fighters within their territories.