Departing from its usual policy of avoiding controversial public statements, Medecins Sans Frontieres (MSF) accused the Russian forces of blocking humanitarian aid, targeting schools, mosques and hospitals with bombs, and firing on fleeing refugees from helicopters.
The agency, which has several bases in the war zone, released the document as world leaders began to arrive in Moscow, saying it was an "urgent call" to the heads of state to pressure the Russians to stop "gross and systematic targeting of civilians and the violations of humanitarian law in Chechnya".
Although the summit itself is about nuclear safety, Chechnya could be raised at bilateral talks with G7 leaders. Mr Yeltsin will also not welcome a public indictment of his failure to settle the 16-month conflict.
Another brutal reminder came yesterday when the Russian military revealed that 53 of its troops - twice the number first reported - were killed in Tuesday's attack on a convoy by Chechen separatists.
"It is not easy for the MSF to make this kind of call," said Dr Eric Goemaere, the agency's director-general, "but we feel that doctors and bandages are not enough to stop this massacre any more." The agency had "very strict rules" about speaking out, but concluded it had no choice but to do so.
Mr Yeltsin insists that full military operations in Chechnya came to an end nearly three weeks ago, after he unveiled his plan to end the conflict before Russia's presidential elections in June. He now only acknowledges there are continuing "special operations" directed at Chechen "bandits".
This is hotly disputed by the MSF, which says village after village in southern Chechnya is being bombed in a war being waged "behind closed doors". It says civilians are being singled out for attack in violation of the Geneva Convention and other international agreements. "There are as many women and children as there are men (among the victims)," said Dr Goemaere. "Can you say that a 12-year-old, or a 60-year-old babushka [grandmother] are bandits? I don't think so."
However, it is uncertain whether the MSF's report, Civilians Targeted, Humanitarian Law Flouted in Chechnya, will prompt significant action from the G7 leaders.
The Foreign Office said that Mr Major will raise Chechnya with Mr Yeltsin at a meeting later today, but MSF representatives say that when they recently tried to raise the alarm about the bloodshed in Chechnya by contacting foreign embassies in Moscow, including Britain's, they were told to keep quiet.