At least 60 people have died in a magnitude 7.3 earthquake that collapsed buildings and triggered landslides in Nepal, India and Tibet.
Nepal's National Emergency Operation Centre said at least 42 people were killed in the country and 1,100 more were known to be injured.
The death toll in India stood at 17 this afternoon and the number could rise.
A school was reportedly among the buildings that collapsed in Bihar state, while Uttar Pradesh also saw a fatality, the Hindustan Times reported.
In Tibet, a landslide triggered by the quake killed one person and injured three, according to China's state broadcaster China Central Television.
Terrified crowds fled buildings as the earthquake struck in Kathmandu, which was already badly damaged, and it could be felt as far away as Delhi and Dhaka, in Bangladesh.
In the Indian capital, offices were evacuated as the ground shook and high rises swayed from side to side.
The earthquake struck near the Everest base camp at around 8am BST (12.35 local time) and was measured at a shallow depth of about 19 km (12 miles).
A series of intense aftershocks of up to magnitude 6.3 followed over the next hour, spreading panic.
The US Geological Survey initially measured the earthquake at magnitude 7.1 but later increased the strength, recording the epicentre near the Himalayan town of Namche Bazar.
Rescue helicopters were sent to the worst affected districts of Sindhupalchowk and Dolkha, where landslides and collapsed buildings were feared to have trapped people.
Second major earthquake hits Nepal
Second major earthquake hits Nepal
1/23 Second earthquake hits Nepal
Medics treat an injured person at Police Hospital in Kathmandu following a further major earthquake in Kathmandu
2/23 Second earthquake hits Nepal
A rescuer carries a man on his back as authorities carry out rescue operations after a fresh 7.3-magnitude earthquake in Gyirong county, Tibet
3/23 Second earthquake hits Nepal
Medics carry an injured person at Police Hospital in Kathmandu following a further major earthquake in Kathmandu
4/23 Second earthquake hits Nepal
Nepalese rescue workers at the site after a house collapsed during a strong earthquake, in Kathmandu
5/23 Second earthquake hits Nepal
Nepalese armed police force search for victims after a house collapsed during a strong earthquake, in Kathmandu
6/23 Second earthquake hits Nepal
Local residents evacuate onto a street minutes after an earthquake in central Kathmandu
7/23 Second earthquake hits Nepal
Local residents evacuate from a shop during an earthquake in central Kathmandu
8/23 Second earthquake hits Nepal
An Indian bystander carries an injured youth who fell downstairs at Siliguri Hospital as a tremor struck in Siliguri
9/23 Second earthquake hits Nepal
Nepalese military personnel and International rescue check on a collapsed building after an earthquake in the centre of Kathmandu on 12 May
10/23 Second earthquake hits Nepal
Nepalese military personnel stands on a collapsed building after earthquake in centre of Kathmandu
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Indian hospital staff attend to a schoolgirl who fainted as a tremor struck at Siliguri Hospital in Siliguri
12/23 Second earthquake hits Nepal
Nepalese run for open space as a strong earthquake hits Kathmandu
13/23 Second earthquake hits Nepal
Nepalese search for open space as a strong earthquake hits Kathmandu. Another quake that hit Nepal had a magnitude of 7.3, the United States Geological Society says, and was followed by several strong aftershocks
14/23 Second earthquake hits Nepal
People run for open space as a strong earthquake hits Kathmandu in Nepal on 12 May
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Nepalese people rush to safer areas as an earthquake hits Kathmandu
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A Nepalese man runs to safety after a second earthquake hit Nepal in Kathmandu
17/23 Second earthquake hits Nepal
A man carries children to safety in Kathmandu
18/23 Second earthquake hits Nepal
Nepalese people stand in the streets as a 7.3 magnitude earthquake hits the country, in Kathmandu
19/23 Second earthquake hits Nepal
People run to safety in Kathmandu
20/23 Second earthquake hits Nepal
Nepalese patients are carried out of a hospital building as a 7.3 magnitude earthquake hits the country, in Kathmandu on Tuesday
21/23 Second earthquake hits Nepal
Families ran into open spaces as the earthquake hit Kathmandu
22/23 Second earthquake hits Nepal
Nepalese patients lie on stretchers after being carried out of a hospital as a 7.3 magnitude earthquake hit Kathmandu on Tuesday
23/23 Second earthquake hits Nepal
An Indian woman being evacuated from the Indra Bhawan commercial complex as the earthquake was felt in Allahabad, India
Yeeshu Shukla, Christian Aid's emergency programme officer, was in Sindhupalchowk, around 40 miles from the epicentre, when the earthquake hit.
"For a moment, I felt that the building I was in would come down," he said.
"We rushed out. Everyone was out on the street, some of them panicking, with mothers screaming, looking for their children. There were four or five severe aftershocks and some buildings collapsed."
Mr Shukla described the roads towards Kathmandu lined with people too scared to re-enter buildings.
Video from the Canadian Red Cross
Aid workers recorded the first fatalities in the remote Nepalese town of Chautara.
Paul Dillon, a spokesperson for the International Organisation for Migration, said several buildings had collapsed and search and rescue teams were already searching through the rubble.
Chautara had become a hub for humanitarian aid in the wake of the last disaster, with dozens of relief workers now based there to send help deeper into the countryside.
Norway's Red Cross, which was helping victims of the previous earthquake at a 60-bed hospital in the town, said on Twitter that there were “many injured, several killed” and added that their hospital tents were taking patients.
There is a sense of panic in the dense downtown area of Kathmandu because of this latest quake. pic.twitter.com/VGOCWSek5b— Sergio Quintana (@svqjournalist) May 12, 2015
An emergency worker speaking to Sky News from the Nepalese capital said the earthquake was "very distressing" and put increased pressure on aid workers.
Olle Castell, the response manager at charity Plan UK, said the quake lasted around 30 seconds and felt like an "aftershock".
"Everyone is very worried and under emotional distress," he added. "Things were shaking and I could see the trees moving like it was a strong wind but there is no wind at all."
At the Norvic Hospital in the Nepalese capital, patients and doctors rushed out of the shaking building into the car park.
“I thought I was going to die this time,” said Sulav Singh, who rushed with his daughter into the street in the suburban district of Thapathali. “Things were just getting back to normal, and we get this one.”
Rose Foley, a Unicef official also based in Kathmandu, said: “The shaking seemed to go on and on. It felt like being on a boat in rough seas.”
She added that the agency was thinking about children across Nepal, who are already suffering and could be made more vulnerable.
Today's tremors came after the 7.8-magnitude earthquake that struck on 25 April, killing at least 8,046 people and injuring more than 17,800 more.
The Foreign and Commonwealth Office said it was investigating whether any British nationals were affected.
Additional reporting by agenciesReuse content