"In the name of the government and in agreement with the president of the republic, I have today ordered our armed forces to conduct the war and recover regions currently under occupation and restore the authority of the state," Kengo wa Dondo said on state television.
"The sacred duty of every Zairean man and woman is to defend and maintain the nation's territorial integrity," he added.
The rebels, who launched their offensive last October, hold towns and territory in the east along the borders with Rwanda, Burundi and Uganda after routing the Zairean army there. The counter-thrust against them has been promised for more than a month.
Zairean troops and mercenaries have been gathering in the eastern town of Kisangani. Three Russian-built combat helicopters, bristling with rockets and 24mm cannon, circled over the town yesterday. Witnesses report regular arrivals of planes in recent days, with weapons and men. The army chief of staff, General Mahele Leiko Bokoungo, appointed in December with wide-ranging powers, arrived last week.
President Mobutu Sese Seko returned home last month after cancer surgery and four months' rest in Europe and reshuffled the government and army leadership to confront the rebels. He has since returned to Europe for more tests.
Security sources said yesterday that the military commander of the rebels in eastern Zaire had been killed in an ambush. After days of rumours in rebel-held areas about Andre Kisase Ngandu, the sources said he was killed on or about 8 January near the rebel-held town of Butembo.
There was no official confirmation from Laurent Kabila, chairman of the rebel Alliance of Democratic Forces for the Liberation of Congo-Zaire.