RWANDAN soldiers shot dead a Congolese soldier who crossed the border and shot at security forces, triggering a brief exchange of fire between the two sides, Rwanda's defence ministry said. Already strained relations between the central African neighbours have deteriorated sharply since the M23 rebel group launched a fresh offensive in eastern Congo in March last year, displacing over 600,000 people since. Congo accuses Rwanda of backing the Tutsi-led group. The United Nations, the United States, France and other international powers have made similar allegations. But Rwanda denies this and says it is being scapegoated for Congo's military incompetence. The…