Damage to infrastructure is hampering the rescue effort after an earthquake struck Nepal and killed thousands of people this weekend.
Buildings have collapsed across the mountainous region, and roads have been blocked.
The quake, reported to measure 7.8 magnitude, struck the Asian country on Saturday 25 April.
The Red Cross said up to 6M people could be affected, across a 100km area. It added that forecast rain could make the dire situation even worse.
It launched an appeal to help bring in support including food, drinking water, emergency shelter and medical equipment.
British Red Cross director of programmes Alexander Matheou said: “The impact of this powerful earthquake has been devastating.
“People are now sleeping out in the open, too scared to return home while the aftershocks go on. Some have no home to return to.
“Search and rescue efforts are continuing, but some roads, especially in areas near the epicentre, have been damaged or blocked by landslides or rubble. We are extremely worried about these communities.”
The Red Cross has been working on an earthquake preparedness programme with 66 communities across the Kathmandu valley.
It said this meant hundreds of thousands of people had been given earthquake preparedness information, and thousands more had been trained in first aid as well as light search and rescue techniques.