Authorities in western Indonesia say a shortage of available boats are hampering their ability to bring in needed supplies in the aftermath of a deadly tsunami that has killed more than 400 people.

The tsunami roared across the Mentawai Islands on Monday, destroying villages and sweeping untold victims out to sea.

On Friday, Agus Prayitno of the West Sumatra provincial disaster management centre said officials had confirmed the deaths of at least 408 people since the tsunami hit four days before. Another 303 people remain unaccounted for and are suspected to have been swept away.

Relief efforts are now based in nearby West Sumatra, though a lack of suitable roads means that all needed supplies need to be brought in by boat or helicopter.

Suryadi, the official co-ordinating relief in West Sumatra, said boats were initially unable to reach the islands because of bad weather. But even with improved weather conditions on Friday, there are not enough boats available to get the job done because many were lost in the tsunami.

"We need more boats," Suryadi said Friday, without providing a hard figure on the number of needed boats.

In Pagai Utara, one of the islands hit by the tsunami, survivors have filled a small hospital that is treating a two-month-old boy who was found in a storm drain after the disaster.

Inside the same hospital, a five-year-old boy named Dimas was treated for a broken arm on Friday. His father Sarifinus said he had tried to flee the tsunami with Dimas' two young brothers, but the massive wave tore them from his grasp and sucked them into the water. Dimas was found alive.

The tsunami and the 7.7-magnitude earthquake that triggered it weren't the only disasters to hit Indonesia this week.

A volcano in central Java erupted less than 24 hours after the tsunami, killing 33 people who lived near Mount Merapi, about 1,300 kilometres east of the Mentawai Islands.

Further eruptions followed on Thursday and Friday, though no further deaths were reported. Local scientist Safari Dwiyono said the additional activity was helping to ease pressure inside the volcano.

"If the energy continues to release little by little like this, it reduces the chances of having a bigger, powerful eruption," he said.

The people who had to flee their homes because of the eruption are now living in refugee camps. Officials say 13,000 people are homeless.

Twenty-six of the volcano victims were buried at a site about 10 kilometres away from the base of the volcano. Thousands of local residents attended a service for the dead and watched as their bodies were put into the ground.

With files from The Associated Press