ABC News' Terry Moran reports:

BANGUI, Central African Republic - What has been unleashed here in the Central African Republic is terrifying.

Since December, about 4 million people have made war on each other across religious lines, Muslims attacking Christians, Christians responding with unspeakable ferocity.

Samantha Power, the U.S. ambassador to the United Nations, visited today for the second time in an attempt to mobilize the world to prevent a genocide.

(Photo Credit: Jerome Delay/AP Photo)

"I think it's our job to, we hope, to raise the resources that we need to help the people facing this kind of peril," she said.

The images are startling: a boy chasing a man with a machete, a woman weeping over the coffin containing a child, a man raising a club to beat an unseen victim.

No one knows the death toll because the killing is too frenzied and too widespread to count. In the heart of Africa, CAR is one of the poorest countries in the world.

Up to 700,000 people - almost 20 percent of the total population - have been driven from their homes by warring militias and nearly 100,000 innocent people have gathered at the main airport.

Conditions are deplorable. Families need food and water. People are sick and are stuck under the broiling sun. They have been forgotten by much of the world.

Across a barbed-wire fence - put there for their safety, officials say - people told ABC News, in French, how they fled for their lives the night the killers came, brandishing machetes and knives with glee.

There are several thousand international peacekeepers but they are overwhelmed, as 25 of them have been killed and others have allegedly joined in the mass killings.

The U.N. Security Council is expected to vote Thursday on expanding the African mission in the country into a U.N. peacekeeping operation.