Iran has been struck by its most powerful earthquake for nearly 40 years, with tremors felt across Pakistan, India and the Middle East.
The epicentre of the 7.8-magnitude quake was near the south-east city of Khash, close to the Pakistani border, the US Geological Survey said.
Offices were evacuated in Karachi, Delhi, and in the Gulf states.
There were conflicting reports on casualties. Iranian state TV said at least five people had been injured.
It denied earlier reports that 40 people had been killed.
At least five people died in the Pakistani town of Mashkel, near the border with Iran, hospital and local government officials said. Some reports said more than 20 people had died there.
The earthquake struck in the province of Sistan Baluchistan at about 15:14 local time (10:44 GMT), close to the cities of Khash, which has a population of nearly 180,000, and Saravan, where 250,000 live.
“The epicentre of the quake was located in the desert, and population centres do not surround it. There were no fatalities in the towns around the epicentre,” an Iranian crisis centre official, Morteza Akbarpour, was quoted as saying by the Iranian news agency Isna.
Iran’s Red Crescent said it expected limited damage and a low death toll because the earthquake was so deep – the Iranian Seismological Centre estimated the depth at 95km (59 miles).
However, one unnamed Iranian government official told Reuters: “It was the biggest earthquake in Iran in 40 years and we are expecting hundreds of dead.”
All communications to the region have been cut, and the Iranian Red Crescent said it was sending 20 search-and-rescue teams with three helicopters to the area.
Sistan Baluchistan is Iran’s biggest province and one its most impoverished areas.
A member of parliament for Saravan, Hedayatollah Mir-Morad Zehi, said there were 1,700 villages in the area, and most of the buildings were made of mud.
Many people in surrounding villages live in tents, which it was hoped would limit the number of casualties.
Iran’s Fars news agency reported that Saravan had suffered no serious damage.
In the Pakistani town of Mashkel, which has a population of about 45,000, communications were also disrupted. But aid workers said many houses were thought to have been damaged or destroyed.
The earthquake was felt across the region.
Michael Stephens, a researcher at RUSI Qatar, told the BBC from his office in Doha: “I definitely felt the walls shaking. It lasted for about 25 seconds.”
Mohammad Wazir, a correspondent for BBC Persian in Pakistan, says the quake was felt in the cities of Karachi and Quetta.
Tuesday’s earthquake was about 180 times stronger in energy release than a 6.3-magnitude quake that struck on 10 April near the nuclear plant at Bushehr in south-western Iran. That quake killed at least 37 people and wounded 850.
The Bushehr plant was not damaged by the earlier earthquake, and an official at the Russian firm that built the plant said it had not been damaged by Tuesday’s earthquake either, Reuters reported.
Scientists say earthquakes in south-eastern Iran are triggered by the clash between the Arabia and Eurasia tectonic plates, the former of which is pushing north at a rate of several centimetres each year.
In 2003, a 6.6-magnitude quake destroyed much of the south-eastern city of Bam and killed some 26,000 people.