MiG-15 is the most produced jet fighter in the world with more than 13 000 built in USSR alone (5 factories were producing it simultaneously) and more than 4 000 in Poland, Czechoslovakia and MiG-15UTI in China. It was used by many countries and is believed to be still in use by DPRK as a trainer aircraft. Initial MiG-15 was powered by RD-45 engine that was a copy of Nene II engine bought in UK (it is interesting to note that American F9F Panther also used the licensed copy of this engine), while MiG-15bis received more powerful VK-1 engine that had a number of improvements. In Korea, MiG had more powerful weaponry than its main adversary: three cannons (37 mm N-37 and two 23 mm NR-23) versus the six 12.7 mm (.50 inch) machine guns of the American F-86 Sabre.