KATHMANDU, Sept. 3 (Xinhua) -- The mud was knee-deep at first. Then it rose to the rescuers' chests.

About 1.8 kilometers into a tunnel in Bidur, Nuwakot District, Nepal, Zhao Bo, deputy commander of the Chongqing-based national tunnel emergency rescue team, which is run by the China Communications Construction Company, said they could no longer walk. Their legs sank into sediment and could barely be pulled free. "We had to crawl our way forward."

Zhao is one of the Chinese rescue experts now racing against time to search for possible survivors in areas hit by the deadly mudslide in Nepal, where roads have been cut off and tunnels have been buried under thick layers of mud and sediment.

The team eventually reached about 2.3 km from the tunnel entrance. They shouted into the darkness, knocked repeatedly and blew whistles, waiting for any response.

For nearly 20 minutes, there was none.

Zhao said that the team had traveled about 2.5 km into the tunnel, but the passage farther ahead was almost entirely buried in mud. To conserve their strength, they eventually turned back.

The search is not over. It is only the beginning of a rescue operation constrained by mud, water, blocked roads, limited helicopters and difficult mountain weather.

On Sunday morning, two Chinese Y-20 transport aircraft carrying rescue supplies arrived at Kathmandu International Airport, bringing four Chinese tunnel rescue experts. Five other Chinese tunnel rescue experts had arrived a day earlier to assist with the search and rescue operation in Nepal.

At a Nepalese rescue command center in Bidur, helicopters take off and land almost continuously as rescue operations proceed at a tense and urgent pace.

But the number of helicopters available is limited, while demand for aerial rescue and transportation is enormous. Chinese experts have to coordinate with the Nepalese side to use the available aircraft.

Time is another major constraint.

For safety reasons, each tunnel exploration mission could last no more than two hours. Meanwhile, Nepal's rugged mountainous terrain and complicated weather conditions sharply limit the daily time helicopters can operate safely.

The difficulties are even more severe inside the tunnels.

On Sunday afternoon, two Chinese rescue experts returned from a search mission in a tunnel.

Exhausted from the operation, they nevertheless sat down and immediately began reviewing video footage recorded inside the tunnel with their colleagues, assessing the conditions and discussing the next rescue plan.

Li Yingping, an expert from the Kunming-based national tunnel emergency rescue team run by the China Railway No. 2 Engineering Group Co., Ltd., described the tunnel as being filled with sediment.

"The mud has become like porridge," Li said.

The mud, about 2 meters deep, could not support a person's weight and was also unsuitable for a small rescue boat, he said.

Pan Jiangyuan, another tunnel rescue expert who also participated in the Sunday afternoon operation, said insufficient transport capacity was another major obstacle facing the rescue effort.

Pan said that the only available means of transport at present are helicopter air drops and manual carrying, making it difficult to get certain rescue equipment and instruments to the site.

Despite these obstacles, the rescue teams have continued to move toward areas that ordinary rescue vehicles cannot reach.

To speed up rescue efforts, every Chinese search mission is conducted jointly with Nepali military personnel and local project engineers. Nagendra Bahadur Bhandari, a Nepali military rescuer who worked with Pan, said that they cooperate very well with each other.

On Monday, eight Chinese rescue experts flew to the disaster zone in two groups for another round of operations.

Near their target area, they spotted three Nepalese people waiting to be rescued.

The Chinese team immediately guided them toward a safe platform where a rescue helicopter could land. The three people were subsequently rescued by the Nepalese military.

Nepalese authorities said Tuesday that the death toll from massive mudslides had risen to 1,010, with 4,599 people still missing.

"The Chinese rescue team stands ready to explore every last corner, no matter how tough the situation gets," Zhao said.