After six days of digging troops have finished a drainage channel
A helicopter evacuating injured people from China's earthquake zone has crashed in fog with 14 people on board.
An operation to look for survivors is under way alongside the wider rescue and relief effort in Sichuan province.
Troops battling to stop a lake formed in the quake from flooding the city of Mianyang are reported to have finished digging a diversion canal.
A new official toll puts the number of people killed in the 12 May disaster at 69,016, with another 18,830 missing.
Many of those who died were children, killed when their schools were destroyed.
Almost 7,000 classrooms are reported to have collapsed, killing more than 11,000 children and teachers, triggering complaints that the schools were badly built.
China has vowed to investigate such charges.
On Sunday, many parents marked International Children's Day by gathering at the sites of collapsed schools to mourn their offspring.
Surviving children living in refugee camps, many of whom have been orphaned, performed in ceremonies marking the usually celebratory day.
Children in refugee camps performed for International Childrens' Day
At least 368,545 people were also injured in the quake, according to the Chinese government, and about five million were made homeless.
People have been further displaced by more than 30 so-called quake lakes - vast pools of water created when the earthquake triggered landslides down plunging valleys, clogging rivers and turning them into fast-rising lakes.
More than 200,000 people have been evacuated to higher ground away from the largest such lake which is threatening to further devastate the hard-hit city of Beichuan before flowing down into the until now largely undamaged city of Mianyang.
Prepared to run
More than 600 Chinese troops working round the clock for six days have finished digging a canal aimed at draining off some of the rapidly rising water.
Soldiers rescue children stranded by earthquake flooding
If dams burst water could reach Mianyang within hours and China has emergency plans to move a further one million people if necessary.
People have been told to be ready to leave in a hurry.
The BBC's Nick Mackie in Mianyang says public transport would be on hand to take children, the old and infirm to some 61 sites around the city that are being equipped with toilets and supplied with food and water.
The majority, however, on hearing the sirens, would have to go on foot as no private cars would be allowed, our correspondent says.
The authorities have tested their communication channels and readiness to respond to a breach in the barrier holding back some 180 million cubic metres of water.
Thousands are under threat if the water spills over from the 'quake lake'
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