Earlier reports said Iran's seismological centre pegged Tuesday's earthquake at a magnitude 7.5, but that was changed to 7.7 later in the day.
The U.S. Geological Survey put the preliminary magnitude at 7.8 and at a depth of 15.2 kilometres.
Press TV in Iran said least 40 people were killed in the latest quake, which flattened homes and offices on both sides of the border Tuesday, but gave no other immediate details on the extent of damage or casualties. Later, the reference to the death toll was dropped from Press TV's website and the news agency IRNA said only that at least 27 people were injured.
The discrepancies and apparent backtracking in the Iranian reports could not be immediately reconciled, but given last week's quake in Iran that killed 37 people, authorities could seek to downplay casualties this time out.
Homes, shops collapse
A Pakistani police officer, Azmatullah Regi, said nearly three dozen homes and shops collapsed in one village in the Mashkel area, which was the hardest hit by the quake. Rescue workers pulled the bodies of a couple and their three children, aged five to 15, from the rubble of one house, he said.
The Pakistani army ordered paramilitary troops to assist with rescue operations and provide medical treatment. Additional troops are being moved to the area, and army helicopters were mobilized to carry medical staff, tents, medicine and other relief items.
In Iran, the Red Crescent said it was facing a "complicated emergency situation" in the area with villages scattered over desolate hills and valleys.
The centre said the quake was centred near Saravan, a sparsely populated area about 48 kilometres from the Pakistani border.
The quake was felt over a vast area from New Delhi — about 1,500 kilometres from the epicentre — to Gulf cities that have some of the world's tallest skyscrapers, including the record 828-metre Burj Khalifa in Dubai. Officials ordered temporary evacuations from the Burj Khalifa and some other highrises as a precaution.
"The epicentre of the quake was located in the desert, and population centres do not surround it," an Iranian crisis centre official, Morteza Akbarpour, was quoted as saying by the Iranian news agency Isna. "There were no fatalities in the towns around the epicentre."
Carla Friesen, a Canadian and a teaching assistant at the Aramco Oil Compound in Ras Tanura, Saudi Arabia, told CBC that she felt the impact of the earthquake while studying at the home she shares with her husband, Randal, and daughter Greta, 10, and son Oliver, 8.
"I was sitting on the couch doing my Athabasca University course [online] and I felt the slight tremor. Oliver felt his desk rumbling at school and his pencil fell off his desk."
A resident in the quake zone, Manouchehr Karimi, told The Associated Press by phone that "the quake period was long," and occurred "when many people were at home to take a midday nap."
Last week's 6.1-magnitude quake hit about 96 kilometres southeast of Bushehr, the site of Iran's reactor.
In 2003, about 26,000 people were killed by a magnitude-6.6 quake that flattened the historic southeastern Iranian city of Bam.