Quake 'worse than expected'
U.N. official: At least 500 confirmed dead
CNN's Hugh Riminton repors on devastation on Nias Island.
Aid workers are finding those affected live in inaccessible areas.
Scientists try to find out why two quakes had such different results.
(CNN) -- Aid efforts to earthquake-ravaged Sumatra are being hampered by the level of destruction, amid reports the situation is worse than expected on the hardest-hit island of Nias.
Many of the roads on the island are impassable and the only link to the airport is cut, restricting the delivery of urgently needed aid, which is starting to arrive on the island 48 hours after the quake struck.
About 600 people have been confirmed dead in Nias from Monday's 8.7 magnitude tremor, but that toll is expected to rise substantially as the extent of the destruction and injuries become apparent.
Australian military aircraft have begun flying additional medics and supplies to the region as the island's only hospital was all but destroyed.
Little or no electricity and fuel is forcing doctors to tend to the injured, many suffering compression and crush injuries, without power.
"Reports that I've had in now overnight are, I must admit, pretty bad," Australian Foreign Minister Alexander Downer told Australia's ABC Radio Thursday.
"There's no doubt about this being a significant humanitarian crisis and the Indonesians have deployed resources very quickly, but we're obviously ready to provide additional assistance if it's needed and it may very well be needed," Downer said.
Indonesia has said it welcomes help and President Susilo Bambang Yudhoyono is expected to arrive on the devastated island today.
Indonesia "welcomes and is open to all kinds of assistance, including help from foreign troops, to assist in the disaster zone," a spokesman for the President told The Associated Press.
CNN's Hugh Riminton, who is on Nias, reports that the smell of decomposing bodies now hangs over the main town, Gunungsitoli, which has been all but obliterated.
In the absence of machinery, residents are using their hands to dig through the debris to search for survivors.
For those left alive, it is becoming a battle just to survive.
Helicopters from the Singapore Air Force landed in a sports stadium in Gunungsitoli, but a crowd looted supplies soon after they were unloaded.
This prompted the U.N.'s operations officer to order no more landings in the stadium.
Three Chinook helicopters ferried out the most critically injured people Wednesday morning, although more casualties continued to arrive for evacuation.
In Washington, U.S. officials said the USNS Mercy hospital ship has been ordered to remain in the Indonesia area to help with earthquake relief. It is expected to reach Nias in about six days from its current station near East Timor.
Indonesian officials estimate that, overall, at least 1000 people are dead following the quake, but expect that toll could rise to 2000.
Monday's epicenter was about 60 miles (100 kilometers) south of the December 26 quake which triggered devastating tsunami waves and left an estimated 300,000 people either dead or missing.
Multiple smaller quakes followed Monday's tremors, including two others of 6.0 magnitude or higher.
Both of those occurred in the early hours after the initial quake, which itself was considered an aftershock of December 26.
Although many towns and villages on Nias were badly damaged, a feared tsunami did not materialize.