Heavy storms and blocked roads are hampering rescue efforts after a powerful earthquake killed around 12,000 people in southwest China.The death toll from the country's worst earthquake in three decades is likely to soar, with thousands of people still buried under rubble in the Mianzhu area of Sichuan province.China said 18,645 people are buried under debris in the city of Mianyang, near Wenchuan county, the epicentre of Monday's devastating earthquake. Some 3,629 people have been confirmed dead.There have been almost 2,000 aftershocks keeping nervous residents on edge. In Chengdu, the capital of Sichuan, office workers fled into the streets on Tuesday during the strongest aftershock of the 7.9 magnitude quake.At least 19 British tourists - clients of the Travel Collection company, which is part of the Kuoni holiday group - are unaccounted for after the quake.But travel organisation Abta said at least 100 other Britons travelling in the severely damaged Sichuan province are safe and unhurt.Spokesman Sean Tipton said: "We have been in contact with UK holiday companies and there are about 100 clients in the Sichuan region at the moment. They are all safe and none are in danger. In line with Foreign Office advice, there will be no tours to that region for the time being."Chinese Premier Wen Jiabao, who rushed to Sichuan, ordered roads to Wenchuan, a hilly area about 62 miles from the provincial capital Chengdu that was completely cut off by the quake, to be opened.But rain and thick clouds meant that military helicopters dispatched to the area could not yet land and a plan to parachute in soldiers was put on hold because of the storms.In the Sichuan city of Dujiangyan - about midway between Chengdu and the epicentre - there are scenes of devastation, with buildings reduced to rubble and bodies in the streets.Rescuers are working frantically, pulling bodies from homes, schools, factories and hospitals demolished by the quake, which rolled from Sichuan across much of China.In the same city, about 900 teenagers were buried under a collapsed three-storey school building. Premier Wen bowed three times in grief as the first 50 bodies were pulled out."Not one minute can be wasted," said Mr Wen, a trained geologist. "One minute, one second could mean a child's life."Frantic relatives tried to push past a line of soldiers surrounding the school, desperate for news of their children. "We're still pulling out people alive, but many, many have died," said one medical worker.At a second school in Dujiangyan, fewer than 100 of 420 students survived, according to reports.The initial huge tremor has been followed by a series of aftershocks. "Some are still very strong," said a Dujiangyan resident. "We have put up tents outside to sleep in."Neighbouring areas were also affected, with 150 reported dead in Longnan city of the northwestern province of Gansu and school collapses in the municipality of Chongqing.In Gansu, the quake caused a train to derail, spilling petrol tanks and sparking a fire.In Sichuan's Shifang, where the quake sparked a major chemical leak, about 600 people died and as many as 2,300 remain buried. Two chemical plants collapsed, causing more than 80 tonnes of liquid ammonia to leak out.In Chengdu, many residents slept outside or in cars as aftershocks were felt through the night in a city where at least 45 people died and 600 were injured.