"The United States, UN, Russia, Uganda and other countries have offered assistance to the national committee that will investigate the crash in which the first vice president died," Ismail told reporters in Khartoum on Tuesday.
A foreign ministry committee has begun compiling "information and statements that will help in the probe," he added.
Ismail meanwhile described as "strange" a reported statement by the Ugandan interior minister that an unidentified body had been found in the wreckage as "the aircraft was Ugandan and took off from Uganda with Ugandan crew members."
The Ugandan presidential helicopter crashed in southern Sudan on 30 July, killing all on board and sparking riots in Sudan that left hundreds dead.
Initial reports suggested that the helicopter had been brought down by bad weather, but there have since been suggestions that foul play may have been involved.
Garang was appointed first vice president barely three weeks before his death as part of a peace deal that ended 20 years of civil war in the country.