Al Jazeera's Amr el-Kahky in Egypt said: "We understand that there were threats if any attempt was made to free these hostages by force - they could be in danger."
El-Kahky said the group was comprised of five Germans, five Italians, two Romanians and seven Egyptians acting as drivers and guides.
While Germany's foreign ministry said only that it had formed a "crisis team", Egyptian officials said Germany was negotiating a ransom with the kidnappers.
Egypt's Middle East News Agency said the ransom demand was as high as $15 million, while Zoheir Garana, the Egyptian tourism minister, has said the tour company that organised the safari was negotiating in the range of $6 million.
Sudanese forces have reportedly been deployed to a nearby area.
"Military forces are in the area, but we are not going to make any move that puts the lives of those being held in any risk," Yousuf said.
"Germany is in contact with the kidnappers, and Sudan is remaining in close contact with the Egyptian, Italian, German and Romanian authorities."
Egypt has said Germany is negotiating through the German wife of the Egyptian tour operator who is among the missing, saying she would be given the ransom to hand over.
Egyptian officials said the abduction took place on Friday, when the tour leader and owner of the company managed to phone his German wife.
She in turn reported the incident to the authorities.
The tour leader told her that a group of armed men who appeared "African" drove up to them while they were pitching their tents.
It was not clear when the phone call took place but it was traced to Sudan.