The first rescuers reached the worst-hit site in Tibet on Friday afternoon, Chinese state media reported.

Their arrival came more than 50 hours after a wave of mud and debris crashed across the border into the trade hub of Gyirong on Wednesday morning.

The death toll in Tibet from the mudslide rose from three to five by Friday afternoon, with 558 people still missing, according to figures released in China.

- Secondary disasters -

Chinese officials have warned of "great difficulties" in the rescue operation because the border region remains at risk of secondary disasters.

President Xi Jinping chaired a meeting of top leaders about the rescue, which they said "is facing great difficulties".

"There are still many hidden dangers of glaciers, ice lakes and geological disasters in the surrounding area," state news agency Xinhua quoted the leaders as saying.

A huge lake formed upstream after the mudslide hit on Wednesday, raising concerns that it could burst and cause further severe flooding.

Rescue operations were temporarily paused earlier on Friday due to risks of "secondary disasters" from the lake overflowing, state broadcaster CCTV said.

It said at the time that around "50,000 cubic metres of ice debris have flowed into the... lake", citing footage captured by a military drone.

Hours later, another CCTV report said risks were "gradually decreasing" as water from the lake was "flowing out naturally, with the outflow discharge exceeding the inflow discharge".

The US Geological Survey said the disaster was caused by a glacial collapse that triggered a vast flow of icy debris.

- Buried roads -

Footage from Chinese media on Thursday and Friday showed roads submerged by mud up to three kilometres (2 miles) from the border, as rescuers climbed over fallen branches and rocks towards the main disaster site.

Rescuers needed to plough through mud at least 1.5 metres thick, according to local media reports on Wednesday.

Officials in Gyirong County asked volunteer groups to refrain from visiting the area to ensure "safe and orderly" rescue operations, according to a statement shared by local media on Friday.

The Tibet Autonomous Region is largely inaccessible to foreign journalists, who are generally only allowed to visit on government-organised trips.

Communications at the worst-hit site remained down on Friday.

- Days of rain -

Heavy rain and scattered thunderstorms through Monday could further complicate the search.

Local weather authorities on Friday said "heavy rain" would fall in Gyirong Town -- where the mudslide struck -- through Saturday.

Rain could lighten on Sunday and Monday but the region could also see thunderstorms, the county's weather bureau said.

"The risks of disasters is relatively high," the bureau warned, because continuous rain could cause flash floods and mudslides.

CCTV said that 499 Chinese nationals and 555 foreigners had been transported out of the disaster zone by Friday.