Despite common ideological roots and considerable Soviet assistance in the past, tensions in relations between China and the USSR had begun to escalate in the mid-1950s. The 1968 Soviet invasion of Czechoslovakia and the buildup of Soviet forces in the Soviet Far East raised Chinese suspicions of Soviet intentions. Sharp border clashes between Soviet and Chinese troops occurred in 1969, roughly a decade after relations between the two countries had begun to deteriorate and some four years after a buildup of Soviet forces along China's northern border had begun. Particularly heated border clashes occurred in the northeast along the Sino-Soviet border formed by the Heilong Jiang (Amur River) and the Wusuli Jiang (Ussuri River), on which China claimed the right to navigate. Border provocations occasionally recurred in later years--for example, in May 1978 when Soviet troops in boats and a helicopter intruded into Chinese territory--but major armed clashes were averted.