As well as the aircraft operated by volunteers against the Japanese over China, SBs were used in combat against the Japanese during the Fighting near Lake Khasan on the eastern sector of the China–Russia border in July–August 1938, one SB being lost. Fighting between Soviet and Japanese forces broke out again at Khalkhin Gol in Eastern Mongolia in May 1939. While SBs were not involved in the May air battles, where the Soviet forces received heavy losses, two Regiments of SBs were deployed to Mongolia in June, flying their first missions on 26 June. SBs were used heavily against Japanese forces when they attacked in early July. The Soviet SB regiments consisted of a mixture of early and later SBs, whose differing speeds caused problems in maintaining formation, while Japanese Nakajima Ki-27 fighters proved adept in exploiting the poor defensive armament of the SB, with the radio operator operating both the dorsal and ventral guns. To minimise losses to Japanese fighters, the Soviets changed tactics, flying SB missions at over 6,100 m (20,000 ft) where it was difficult for the Japanese to intercept. SBs continued to be used against the Japanese as the Soviets and Mongolian forces commanded by Georgy Zhukov carried out a successful offensive until a cease-fire was signed in September 1939.