Army Reserve Centre, Gateshead.
Two men had shouted the violent threats as they drove past the teenage cadets, who weren't in uniform at the time.
The pair, driving in a Vauxhall Zafira, were said to have asked the girls if they were in the army before threatening them with beheading.
Have you seen this car ?
Police have traced the car, which is left-hand drive and appears to have foreign plates, and were trying to establish the identity of the occupants.
The driver was described as black or Asian, in his 40s, and of plump or muscular build with a black bushy beard and wearing dark clothes.
The passenger was also said to be black or Asian, in his 30s, and had short black hair and stubble.
It comes weeks after British police forces warned officers not to wear uniforms on the way to work after an ISIS-style plot to kidnap and behead an officer.
More than 7,000 officers in the West Midlands were warned they were in imminent danger following an anonymous telephone call to their headquarters on December 8.
Other forces urgently reviewed safety precautions and warned officers not to wear their uniform or ID badge when off duty.
The official threat level to officers came amid fears extremists will attempt to copy the murder of Fusilier Lee Rigby, which has led to soldiers being warned not to wear their uniform off duty.
ISIS has previously called on followers to launch 'lone wolf' attacks on the West.
Extra patrols have now been put in place around the Army Reserve centre in Gateshead, Tyne and Wear, formerly the Territorial Army, after the violent threats against the girl cadets.
Other Army Reserve centres have also been alerted since the incident at 9.15pm on Wednesday January 21.
Gateshead Superintendent Richie Jackson said: 'The men made no attempt to make any physical contact with the girls or get out of the car, and shouted the comments while driving away from the scene.
'Inquiries are ongoing to establish the exact nature of what was said during the incident and we have spoken to the two teenage girls, viewed CCTV footage from the area and have identified the vehicle and inquiries are ongoing to trace the driver.
'As a precaution we have notified staff at other Army Reserve centres in the area and have had extra officers on patrol in Alexandra Road to reassure residents.
'Although this is concerning, it is an isolated incident and if anyone has any information that could help us with the investigation I'd ask that they contact us on 101.'
A Ministry of Defence spokesman said: 'We are aware of an incident allegedly involving members of the Army Cadet Force - this is a matter for the police and we cannot comment further. Any witnesses are urged to contact the police.
'We understand the girls were not in uniform at the time of the incident and that the suspects asked them 'Are you in the Army?' before issuing the threats.'