U.S. and Pakistani soldiers load quake victims onto a CH-47 Chinook helicopter near Muzafarabad

As the South Asia earthquake rescue effort moves into high gear, United Nations officials say thousands of victims have still not received assistance and millions in Pakistan and India are now homeless.

Four days after the quake struck, aid agencies say their relief efforts are beginning to gain traction.

Emergency supplies are starting to reach some of the hardest-hit areas, key roads are being cleared and assistance is pouring in from around the world. But the sheer scope of the disaster remains nearly overwhelming.

The official death toll in Pakistan is still hovering around 23,000, but relief workers say it is at least twice that high. More than 1,000 are reported dead in India. According to the United Nations, more than two million people are homeless.

In Islamabad, U.N. resident coordinator Jan Vandemoortele issued an appeal for more than $270 million. "It is a disaster of unprecedented proportions," he said. "It is such that no government or organization can cope with it alone. The magnitude is about four million [people] affected."

Secretary of State Condoleezza Rice made an unscheduled trip to Islamabad, where she met with the president and prime minister.

Washington has made an initial contribution of $50 million for emergency relief. Secretary Rice promised more U.S. assistance for longer-term development programs. "The task of reconstruction and recovery will be large, and the United States will want to support the people of Pakistan as they try to rebuild as well," she said.

U.S helicopter delivering supplies to quake victims in Pakistan

More than 40 helicopters, including eight U.S. military aircraft, are flying relief supplies to the affected areas.

But hospitals say they are completely overwhelmed, with thousands of fresh patients arriving every day. Aid workers say new challenges are springing up.

Major towns and cities in the hardest-hit Pakistani areas of Kashmir and the Northwest Frontier province have no fresh water or sanitation facilities, and outbreaks of disease and illness caused by exposure to the elements are a growing concern.

Temperatures are dropping near the quake's epicenter, where thousands of refugees are camping outdoors with only a few tents or blankets to keep them warm.

Meanwhile search and rescue teams work around the clock to locate survivors who may still be trapped beneath collapsed buildings.

They say time is running out, but they refuse to give up hope. Wednesday afternoon, a Turkish team rescued a 45-year-old women, 105 hours after the earthquake struck Saturday.