Rescue teams backed by army sappers using explosives tried to force their way Tuesday to the remote epicentre of a powerful Himalayan earthquake that killed 67 people in India, Nepal and Tibet. Before the grim search for more victims can even begin, the main challenge is to reach the isolated, mountainous impact zone on the border between India\'s northeastern Sikkim state and Nepal after Sunday\'s 6.9-magnitude quake. Convoys of vehicles carrying rescue workers, medical teams and emergency supplies left the Sikkim capital Gangtok at daybreak Tuesday. But progress was tortuously slow over the narrow, badly damaged roads more often frequented by groups of adventurous tourists heading for Himalayan trekking trails. After covering just a short section of the 60-kilometre (40-mile) route to the worst-affected districts of Mangan and Sangthan, the convoys came to a complete halt, with the path ahead blocked by a huge rockfall. As army engineers drilled holes for explosives to try and blow apart the largest boulders, rescuers could only wait in frustration along with distraught locals trying to get through to relatives -- unsure whether they were alive or dead. \"I\'ve been here for six hours, waiting for the army to clear the road,\" said Pema Doma, 37, who has heard nothing from her parents or 16-year-old son in Mangan since the quake hit. \"I\'d walk if they would let me,\" Doma said. \"The anxiety is killing me. What if he\'s screaming for me? What if he\'s calling for me and I can\'t even hear him?\" Those who did attempt walking around the rockfall were stopped by soldiers, who insisted it was too dangerous to go ahead on foot. \"I know many shortcuts to reach Mangan but the army says it\'s not safe,\" said P. Sherpa, 62, whose son is a student at the North Sikkim Academy, a private school in Mangan. \"So all we can do is sit here and stare at the rocks,\" he said. One army official told AFP it could take up to 48 hours to clear the entire stretch of road to the quake epicentre. One bright spot for the rescue effort was a break in the monsoon weather that allowed a resumption of helicopter flights which had been grounded most of Monday by heavy rains and low cloud. Air force officials said food packages and small medical teams with doctors and paramedics had been air-dropped into Mangan and Sangthan. The death toll from building collapses and landslides in Sikkim stood at 35, but Indian Home Secretary R.K. Singh warned the number could rise as emergency relief workers reached far-flung villages. \"We cannot rule out more casualties,\" Singh told a news briefing in New Delhi. More than 5,000 army troops were mobilised across the state to help clear roads and assist with the relief operation.   The Press Trust of India said 26 tourists, including 15 trekkers, had been rescued and taken to army encampments for their own safety. Hundreds of Gangtok residents spent a second night out in the open, too scared to sleep in homes badly damaged by the quake. Many saw out the night in the city\'s football stadium, slinging plastic sheets over the goalposts or sleeping on the terraces. \"The stadium is our kitchen and bedroom for the night. We\'re honestly just too scared to consider anything else,\" said Amrita Laqandri, 32, as she helped make tea and warm bread on stoves brought from her family home. There was some relief as power, cut off by the quake, was restored, but landline and mobile communications remained erratic, especially in the worst-affected areas. In Nepal, eight people were killed and hundreds of homes destroyed or damaged in the east of the country, where rescuers faced the same problems as their Indian counterparts with rains and mudslides blocking the only highway. Seventeen other people died in the Indian states of Bihar and West Bengal, while China\'s official Xinhua news agency said seven people had been killed in southern Tibet, near the border with Sikkim.