Jakarta: A string of deadly earthquakes that rocked Indonesia's Lombok island this summer killed 555 people and injured nearly 1,500, the disaster agency said Friday, with hundreds of thousands left homeless.

The picturesque island next to holiday hotspot Bali was hit by two deadly quakes on July 29 and August 5. On Sunday it was shaken by a string of fresh tremors and aftershocks, with the strongest measuring 6.9 magnitude. Indonesia's disaster agency said Friday most of the deaths occured in the northern part of Lombok, with several also killed in neighbouring Sumbawa island.

Many victims were killed by falling debris as the tremors rippled across the island, causing widespread devastation. Whole communities were flattened, leaving cracked streets lined with rubble, caved-in roofs and collapsed buildings.

Some 390,000 people remain displaced after the quakes, the disaster agency said.

Aid organisations have vowed to step up humanitarian assistance on the island as devastated residents struggle in makeshift displacement camps.

They warned that access to food, shelter and clean water has been insufficient for some residents displaced by the disasters. "We've deployed troops to isolated villages that are difficult to reach," disaster agency spokesman Sutopo Purwo Nugroho said in a statement.

"There are many villages... that are hard to access with motorbikes. Some aid has to be delivered on foot."

Rebuilding costs are estimated to top seven trillion rupiah ($478 million).

Indonesia sits on the so-called Pacific "Ring of Fire", where tectonic plates collide and many of the world's volcanic eruptions and earthquakes occur.