U.N. Emergency Relief Coordinator Jan Egeland says it might be too late to save thousands of earthquake victims in Pakistan's remote mountain villages. Aid is reaching many of the devastated villages, but delays still plague the massive emergency response.

US soldier unloads relief goods, donated by Greece government for earthquake victims at Chaklala airbase in Rawalpindi, Pakistan

The shock and horror from last Saturday's earthquake are giving way to frustration and anger as relief agencies struggle to meet the enormous demands for emergency aid.

The United Nations has opened a center in the city of Abbottabad to feed thousands of survivors, many of whom have been without adequate food or shelter since Saturday's massive 7.6-magnitude quake.

The U.N. emergency relief coordinator, Jan Egeland, toured the hardest-hit areas and says it might be too late to save people in remote mountain villages that were buried by landslides.

"With no roads in the beginning, with very little international presence, I think it is going as well as it could the first week," he said. "But I fear we are losing the race against the clock in these small villages around these centers…." 

Several key roads leading to Muzaffarabad, the capital of Pakistan-controlled Kashmir, have been cleared of debris, and trucks carrying relief supplies are finally pouring into the beleaguered city.

But aid workers say logistical challenges remain, and delivering urgently needed supplies, such as food and shelter, is still taking longer than they would like.

The country director for the Irish aid agency Concern Worldwide, Dorothy Blaine, says one of the main obstacles now is traffic.

"There are hundreds and thousands of vehicles on the road, not just aid, not just trucks, there are journalists driving up there, worried relatives driving up, and it's very, very slow," she said.

Earlier, another powerful aftershock rattled northern Pakistan, sending thousands of terrified residents of Islamabad back out onto the streets.

No damage was reported, but rescue workers say the tremor may have increased the odds of fresh landslides in the quake-affected mountainous areas northeast of the capital.

Pakistan's official death toll from Saturday's earthquake is up to 24,000, but rescue workers say that number could easily double in the next few days.