A Russian hiker was found safe and well on Thursday five days after going missing while trekking on Indonesia's most active volcano, an official said.

The man set off on Saturday to climb Mount Merapi on the main island of Java, but got lost after trying to ascend the volcano's southern side which is covered in huge quantities of ash and lava from eruptions.

More than 130 rescuers joined the hunt for the 26-year-old and finally tracked him down on Thursday afternoon on a dried-up riverbed, said local rescue agency spokesman Teguh Supriadi.

"Rescuers heard his screams for help on Wednesday night," he said. "He was weak when we found him. Rescuers are now trying to evacuate him."

Search efforts were difficult as the Russian kept moving around as he sought to find a way down off the volcano, said Supriadi.

He had alerted his Indonesian girlfriend that he was lost by sending her text messages, and she then informed the authorities.

Merapi, a 2,900-metre (9,600-foot) volcano in central Java, killed more than 350 people in a series of violent eruptions in 2010.