African Union mediators at peace talks in Nigeria had given both the government and the rebels until 1700 GMT deadline to heed their demands.
"How come that the government troops withdraw from Darfur region so long as they are responsible for maintenance of peace and stability in the region?" Ismail asked on Saturday.
"The government military operations against the rebels will stop only if the rebels pull out of the areas they occupied after 8 April (when a first ceasefire was agreed) and refrain from launching attacks on civilians and public property," the minister said.
The AU, which is negotiating peace talks to end a conflict that has displaced 1.6 million people and killed tens of thousands, said it would report any further ceasefire violations to the UN Security Council for action.
Earlier, the AU commander in Darfur, Nigerian Festus Okonkwo, said his efforts to mediate between government troops and rebels had yielded minimal results and the region was now a "time-bomb that could explode at any moment".
Okonkwo, head of an AU team of 834 ceasefire monitors in Darfur, said: "The quantity of arms and ammunition brought into Darfur to meet [the needs of] the present build-up of troops in the region is (so) astronomical that the issue is no longer whether there will be fighting or not, but when fighting will start."
UN says Darfur is in the throes
of a severe humanitarian crisis
A Sudanese brigade, which normally numbers 600-700 troops, advanced towards Labado in southern Darfur on Thursday, backed by about 200 militiamen, he said.
"From a military point of view, this indicates an offensive, which, if launched, would be prejudicial to the peace process."
Okonwo's assessment was delivered to AU-sponsored peace talks in Abuja that were suspended on Monday by the rebels, who accused the government of launching an offensive.
Surge in violence
UN Secretary-General Kofi Annan said that if Okonwo's assessment was true, "it is a major violation of the ceasefire".
US State Department spokesman Richard Boucher said: "We have been very, very concerned about the violence recently, the violence from both sides."
After years of tribal skirmishes over scarce resources in arid Darfur, the rebels took up arms last year.
They accuse Khartoum of neglect and of using so-called Janjawid militias to loot and burn non-Arab villages. Khartoum denies arming the Janjawid and calls them outlaws.
The UN says Darfur, an area the size of France, is suffering from one of the world's worst humanitarian crises with 2.3 million people in need of aid.