Sudan denies Chad's accusations that it backs the United Force for Change rebels, who were beaten back by Chadian troops after launching a dawn raid. Chad's president has threatened to expel about 200,000 refugees who fled conflict in Sudan's Darfur region.
The US called this an unacceptable breach of Chad's obligations. "We call upon the government of Chad to uphold its responsibilities... to provide protection for these refugees as well as to provide international access to the refugees," State Department spokesman Sean McCormack said in Washington.
On Friday, Chad paraded 160 captives, said to be rebels, in a public square while crowds and soldiers looked on.
"We have taken the decision to break our diplomatic relations with Sudan today and to proceed to close our frontiers," Chadian President Idriss Deby told a rally in N'Djamena.
He warned that the international community had until the end of June to resolve the conflict in Darfur, otherwise they would have "to find another country" to shelter some 200,000 Sudanese refugees in eastern Chad.
Mr Deby said he had ordered all Sudanese diplomats to leave the country.
Chad accuses Sudan of supporting and arming the attackers. Sudan denies this and says Chad backs rebels in Darfur.
The threat to close its borders and expel the Darfur refugees is designed to focus international attention on Chad's internal conflict and its complex ethnic links with neighbouring Sudan, BBC East Africa correspondent Karen Allen says.
But an influx of refugees back into Darfur would put enormous pressure on relief agencies, who are already struggling to gain access, our correspondent adds.
In the past month the security situation has deteriorated in west Darfur, with the World Food Programme warning that if fighting broke out near the border with Chad the refugees could find themselves stranded.
Clashes and mutual accusations between Chad and Sudan have continued despite an accord that the two countries signed in February, aimed at easing tensions.
Jean-Marc de la Sabliere, the UN envoy from France, which has some 1,350 troops in its former colony, said the Chadian rebels came from Darfur.
The UN Security Council urged Chad and Sudan to resolve differences through talks and not support hostile actions.
Earlier on Friday, Chadian Defence Minister Bichara Issa Djadallah said 150 people on the rebel side had been killed in fighting in the town of Adre, near the Sudanese border.
"All attackers withdrew into Sudan," the minister said in an interview with Radio France Internationale.
The rebels have vowed to overthrow Mr Deby before the polls scheduled for next month, which the opposition is boycotting but which Mr Deby says will go ahead.
A spokesman for the rebels told the BBC they had not been defeated and would attack N'Djamena again.
The Central African Republic has also closed its border with Sudan in protest at what it called Khartoum's "aggression".Güncelleme Tarihi: 20 Eylül 2018, 18:16