The head of the U.N. commission investigating human rights abuses in Syria warned Monday that the country is in "free fall" and has become a battlefield where civilians are the main victims of acts of terror, from indiscriminate shelling to rape and sectarian killing.

Paulo Pinheiro told the U.N. General Assembly that "massacres and other unlawful killings are perpetrated with impunity" — most by pro-government forces and some by anti-government armed groups.

He urged the international community to demand a diplomatic solution to the 2 1/2-year conflict that has killed more than 100,000 people.

"The world must hear the cry of the people — stop the violence, put an end to this carnage, halt the destruction of the great country of Syria!," he said.

Pinheiro painted a devastating picture of the war-ravaged country: people fleeing to escape bombs from their own government, a collapsing economy and a conflict spilling across borders, "igniting tensions in the whole region."

"The war remains deadlocked as both sides labor under the illusion that a military victory is possible," he said.

He said 4.5 million people are displaced inside Syria, 18 million remain in their homes and have become the first providers of humanitarian aid to their fellow citizens, and more than 2.5 million are unemployed and struggling to survive.

"The estimated cost of the conflict to Syria's economy is between 60 to 80 billion dollars, a third of its pre-war GDP," he said.

The government is relying on its superior weaponry and control of the skies to maintain control of major cities and lines of communications while hundreds of anti-government armed groups have increased operations in the north and south, Pinheiro said.

Few of these rebel groups are able to fight across several fronts, and the majority are seriously fragmented, with fighters shifting allegiances usually to better-equipped and better-funded groups, he said.

Pinheiro demanded that the government stop using imprecise weapons such as unguided missiles, cluster munitions and "thermobaric bombs" on civilian areas.

He said both sides must stop laying siege to cities and towns and cutting off vital supplies of food, water, medicine and electricity.

"Civilians are the real victims of this prolonged war," Pinheiro said. "Crimes that shock the conscience have become a dreadful daily reality in Syria."

An untold number of men and women have disappeared from checkpoints and the streets, he said, and those freed from detention "are living with the physical and mental scares of torture."

Pinheiro said a significant proportion of casualties are deaths from indiscriminate or disproportionate shelling from mortars that have landed in streets, barrel bombs that have turned homes into rubble and surface-to-surface missiles that have destroyed not only homes but neighborhoods.

"Of extreme concern is whether both government forces and anti-government armed groups are positioning military objectives within civilian areas, exposing residents to attacks by the opposing side," he said.

He said crimes of sexual violence, including rape, have been documented taking place at checkpoints, during house searches and in detention centers.

"That civilians should come under such sustained unlawful attacks should shock your conscience and spur you to action," Pinheiro told diplomats from the 193 U.N. member states. "But it has not. As the conflict drags on, you — and the world — have become accustomed to levels of violence that were previously unthinkable."

The General Assembly has adopted several resolutions calling for an end to the Syrian conflict, but they are not legally binding. The Security Council, whose resolutions are legally binding, has been paralyzed and unable to act because of deep divisions between its Western members who support the opposition and Russia and China, who back President Bashar Assad's regime.

The U.S. and Russia are supporting a new international conference to try to get both sides to agree to a transitional government.

Pinheiro said it isn't enough for "those walking along the corridors of power" to be appalled.

"There is an obligation to do what you must to bring this war to a close," he said.