MADRID (AFP) - Spanish police said Thursday they were searching with dogs in wells for signs of a 40-year-old US woman who went missing five months ago while hiking the popular Santiago pilgrim trail.

Denise Pikka Thiem has not been heard from since April 4 when she sent a message to a friend saying she was at a point on the trail near the northern town of Astorga.

A police source told AFP that 300 officers with dogs and a helicopter were deploying in a search that would focus on wells near where she was last seen.

"Following investigations that we opened at the time of the disappearance, a search is now being carried out in an area near Astorga," a police source told AFP on Thursday.

"Investigators now think that there is a possibility that carrying out a search on the ground will lead to new developments in this investigation."

The Way of Saint James is hiked by hundreds of thousands of tourists and Roman Catholic pilgrims who stay at hostels on their way to the northwestern city of Santiago de Compostela.

Spain's Prime Minister Mariano Rajoy wrote to Thiem's parents in Arizona, saying he "understands the pain of a family that does not know where their daughter is", in a letter dated Aug 24 and seen by AFP.

Rajoy also wrote to US Senator John McCain, who the government said had contacted it about the disappearance.

He reportedly asked for the Federal Bureau of Investigation to be allowed to investigate on the ground.

Spanish police "have various lines of investigation open which we hope will lead to the case being solved", Rajoy wrote.