Two large and shallow earthquakes struck off Indonesia's Sumatra island early Wednesday, the US Geological Survey reported, but no tsunami warning was issued and there were no immediate reports of casualties or major damage. 

Frightened residents fled their homes when the twin offshore quakes rocked Bengkulu city on Sumatra's western coast, an AFP reporter said.

"The first earthquake was quick...but another one hit shortly after and it was just as strong," Bengkulu resident Jumentrio, who goes by one name, told AFP. 

"My children screamed hysterically." 

The area was hit by a string of aftershocks, according to Indonesia's Meteorology, Climatology and Geophysics Agency. 

"But until now we haven't received any report of damages due to the earthquakes," said Rahmat Triyono, head of the agency's tsunami and earthquake centre. 

The quakes, of magnitude 6.8 and 6.9, struck within six minutes of one another from 5:23 am (22:23 GMT Tuesday) at a depth of between 22 and 26 kilometres, respectively. 

Shallower quakes tend to cause more damage than deeper ones. 

But The Indian Ocean Tsunami Warning and Mitigation System said there was "no threat to countries in the Indian Ocean". 

There was a "low likelihood of casualties and damage", the USGS added. 

The Southeast Asian archipelago experiences frequent seismic and volcanic activity due to its position on the Pacific "Ring of Fire", where tectonic plates collide. 

In 2018, a 7.5-magnitude quake and subsequent tsunami on Sulawesi island left more than 4,300 people dead or missing. 

A devastating 9.1-magnitude quake struck off the coast of Sumatra in 2004, triggering a tsunami that killed 220,000 throughout the region, including around 170,000 in Indonesia.