Thousands of Pakistanis have been affected by a huge earthquake in Iran that damaged hundreds of homes and killed at least 41 people, sparking a military rescue effort in the remote region.

The United States also offered aid after the 7.8-magnitude quake, Iran's most powerful in five decades, damaged an estimated 2,000 mud-built homes of Mashkail, a town in the poor Pakistani province of Baluchistan.

Tuesday's earthquake was felt across the region and though the epicentre lay in southeast Iran, all but one of the deaths reported so far have been across the border in Pakistan.

Efforts to help the survivors have been hampered by Mashkail's remote location -- communities are scattered, there are no paved roads, no electricity and limited mobile phone coverage, and no proper medical facilities.

Only three tents were visible in the town and frightened families prepared to spend a second night out in the open, sheltering under trees, an AFP reporter said, too scared to return to their homes for fear of aftershocks.

A 5.7-magnitude tremor early Wednesday frayed nerves even further. Pakistan's National Disaster Management Authority said 105 injured people had been treated.

Esa Tahir, a local coordinator for the charity Islamic Relief said that after surveying two local councils in Mashkail, 5,000 people have been affected and around 2,000 mud homes have been damaged or destroyed, along with 150 shops.

At least five government buildings, including administrative and revenue offices, a school and a hospital, were also damaged, an AFP reporter said.

Eight injured people, including four young children from the same family, were waiting for a helicopter to fly them to Quetta, the provincial capital.

A helicopter came but could not land due to a dust storm, a military officer said, adding that they will be evacuated on Thursday.

Some 15-20 people made the boneshaking, 45-minute journey across bare ground to the Iranian border to reach a hospital and see a doctor, according to officials with Pakistan's paramilitary Frontier Constabulary.

While some survivors offered prayers for the dead, others dug through rubble with spades and even knives to try to recover their belongings.

"We often feel tremors here, but this was the worst I've ever seen in my life. I thought a bulldozer was passing by close to my house," Abdul Ghaffour, who is about 50, told AFP.

"Thank God my family and I are safe."

Major General Obaidullah Khattak of the Frontier Corps, another Pakistani paramilitary force, said 16 badly injured people had been taken by helicopter to Quetta for treatment and nine doctors were on the scene.

The area's scattered population made determining the death toll difficult, but Frontier Corps Major Attiq Minhas told AFP at Dalbandin airport, around 250 kilometres (155 miles) from Mashkail, that at least 40 people had died.

On the Iranian side of the border, one woman was reported killed by falling rocks and the Red Crescent rushed 400 tents to shelter some 1,700 people who lost their homes in the quake.

Baluchistan, an inaccessible province bordering Iran and Afghanistan, is plagued by Islamist militancy, attacks on the Shiite Muslim minority and a separatist Baluch insurgency.

Putting aside America's longstanding enmity with Iran, and its more recent strains in relations with Pakistan, US Secretary of State John Kerry offered condolences and assistance with relief work.

Disaster relief contributed to an earlier thaw in relations between the United States and Iran, which accepted US personnel following the Bam earthquake in 2003, which killed more than 26,000 people.

The United States has also engaged in disaster diplomacy with Pakistan, briefly improving its abysmal image in the country following a 2005 earthquake in Pakistani-administered Kashmir, where more than 73,000 died.

United Nations chief Ban Ki-moon also expressed condolences after Tuesday's disaster and said the UN too stood ready to help if needed.

The quake struck in the afternoon with its epicentre around 80 kilometres (50 miles) east of the city of Khash, in the Iranian province of Sistan Baluchistan.

It came a week after another struck near Iran's Gulf port city of Bushehr, killing at least 30 people. The UN's atomic agency said there was no damage then or Tuesday to Iran's nuclear power plant at Bushehr.