Kathmandu (AFP) - An American woman who went missing in Nepal after arriving to help earthquake victims has been murdered, police said Sunday.

Dahlia Yahia went missing on August 7, three days after arriving in the popular tourist town of Pokhara, 210 kilometres (130 miles) west of the capital Kathmandu.

Police on Thursday arrested a man who admitted during questioning to killing the 27-year-old who had been travelling on a tourist visa, said senior local officer Govinda Adhikari.

"He confessed that he killed her and then threw her body into a river," Adhikari told AFP. He declined to give details of how Yahia, who lived in Texas according to her Facebook page, was murdered.

"We have not found the body of the woman yet."

Adhikari said the investigation was continuing, including efforts to determine the motive for the killing.

Yahia's family last heard from her via the messaging service WhatsApp from Pokhara on the night of August 6, according to Nepal's trekking agencies' association.

The family, who reached out to the association to help find her, said she visited a village in Gorkha district, the epicentre of the April quake, before heading to Pokhara.

Scores of foreign volunteers arrived in Nepal to help with relief efforts following the April 25 quake that killed more than 8,800 people and left thousands more homeless.