Chad has accused Sudan of seeking to destabilise it by recruiting Chadian nationals into militias operating in the troubled Sudanese region of Darfur.
The 'Janjaweed' militia are accused of ethnic cleansing
Chad's government spokesman said the 3,000-strong force operated just 25km (15 miles) from their common border.
A two-year conflict between Sudanese pro-government Arab militias and black African rebels has left at least 180,000 people dead in Darfur.
The Chad border region has a similar ethnic make-up to Darfur.
Some 200,000 Darfur refugees, mostly non-Arabs, have escaped to Chad.
Some 1.8 million more have fled their homes but remain inside Sudan.
Chad's Communications Minister Barthelemy Natoingar Bainodji said Arabs from Chad had been recruited to join the Janjaweed militias accused of widespread atrocities in Darfur.
"Chad's government wants to call on the international community to witness the seriousness of the situation. It is an unfriendly act," he said.
Chad President Idriss Deby is from the Zaghawa ethnic group, which is one of those fighting the Janjaweed in Darfur.
Chad has tried to mediate in the conflict between the Darfur rebels and the Sudan government.
It brokered a ceasefire a year ago but this has been frequently violated.