Major earthquake strikes south-east Iran
The epicentre of the 7.8-magnitude quake was near the south-eastern city of Khash, close to Pakistan.
The quake struck deep and in a remote region, apparently limiting casualties.
Iranian state TV said 27 people had been injured, but rowed back on early reports of deaths. However, more than 30 people were killed in Pakistan.
The Pakistani military has been mobilised to help with rescue efforts, officials said.
Two military helicopters carrying medical teams have been sent to the area and troops will support the relief efforts, they said.
The border area has since been shaken by several strong aftershocks.
UN chief Ban Ki-moon said the organisation stood ready to help "if asked to do so". The US has also offered assistance.
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 people 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.
The power of the tremor led to offices being evacuated in Karachi, Pakistan, in the Indian capital of Delhi, and in several Gulf cities.
Iranian state TV initially reported that 40 people had been killed, and one Iranian official was quoted by Reuters news agency as saying hundreds of deaths were expected.
Iran's Fars news agency said the depth of the quake had reduced its impact to the size of a magnitude-4.0 tremor on the surface.
Iranian scientists said it was the country's strongest earthquake for more than 50 years.
All communications to the region have been cut, and the Red Crescent said it was sending 20 search-and-rescue teams with three helicopters to the area.
A resident of Saravan, Yar Ahmad, told BBC Persian that a number of people in the nearby village of Lolokadan had been injured, with broken arms or legs, but only had first aid kits for treatment.
No rescue workers had arrived, and the roads were in poor condition, he said.
Tents and shacks Sistan Baluchistan is Iran's biggest province and one of 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 the area live in tents or shacks, a factor which is thought to have limited the number of casualties.
Across the border in Pakistan, up to 34 people were killed and about 80 injured in the Mashkel district of Balochistan province, the army said.
Communications were disrupted in Mashkel, which has a population of about 45,000. 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.