The government of Chad gained tacit support from the United States as the US State Department warned Sudan against any possible attacks on Chad.
Chadian Foreign Minister Ahmat Allami claimed the Sudanese government was "reforming a new army" to attack Chad, after the failure of an assault by insurgents on the Chadian capital N'Djamena last week which left some 400 dead.
"Preparations are under way on the other side of the border. The Sudanese are reforming a new army. The Sudanese are preparing a new massacre," Mr Allami told news agency AFP by telephone.
The office of Chadian President Idriss Deby Itno released a statement accusing Sudan of conscripting young people in Darfur to join the Chadian rebels and of sending them weapons to use in a new attack.
"According to corroborating sources, the Sudanese government is carrying out a forced recruitment of young people in Darfur to replace those who died during the latest combats," his office said in a statement.
Chad broke off diplomatic relations with Sudan last Friday, a day after the attack on N'Djamena by the rebel United Front for Change (FUC), which is seeking to topple President Deby.
Washington stopped short of officially endorsing Chad's allegations but a senior State Department official, who asked not to be named, hinted that it suspected there may have been some involvement by the regime of Sudanese President Omar al-Beshir.
Chad initially threatened to retaliate by expelling the 200,000 refugees from the civil war in Sudan's western Darfur region now living in camps in Chad.
But on Monday the UN refugee chief Antonio Guterres said President Deby had assured him that "refugees will not be refouled (forcibly returned) and Chad will abide by international principles".
"President Deby expressed his understandable concern about the difficulties involved in providing security both to the refugees and to the humanitarian organisations that are helping them," the UN High Commissioner for Refugees said in a statement.
Chad has been a co-mediator in the effort to bring peace to Darfur, where three years of fighting between rebels and Khartoum-backed militias have left up to 300,000 people dead and two million displaced.
But the Chadian government considers the Darfur conflict to be the main destabilising force in Chad, and views the refugee camps in the east of the country as recruitment "reservoirs" for the FUC rebels.
On Sunday Chad withdrew its delegation from Darfur peace talks between the Khartoum government and the rebels in the Nigerian capital Abuja.
The rebel offensive in Chad has triggered alarm in the international community and comes just weeks ahead of planned presidential elections in the oil-rich but impoverished state.
President Deby's regime has also been wrangling with the World Bank over Chad's nascent oil industry, after the Bank had Chad’s oil revenue bank account in London frozen in January.
The Bank's actions, triggered by complaints that Chad had circumvented requirements to earmark part of the revenues for social programmes, prompted a government threat to shut down oil production.
Washington said it was sending its deputy assistant secretary of state for African affairs, Donald Yamamoto, to N'Djamena at the end of the month to mediate in both the oil row and the standoff with Sudan.
Meanwhile, tensions were reported in several areas of Chad on Monday in the wake of recent clashes between the FUC and government troops.
Sources on both sides also said several officers in the Chadian army had deserted to the FUC in recent days.