Police are still looking for a Washington man they believe murdered his three young daughters one week ago.

Travis Decker, 32, is wanted on kidnapping and first-degree murder charges for killing his daughters, aged nine, eight and five, according to the Wenatchee Police Department in Washington state.

Officials believe Mr Decker, who they say is an outdoorsman and may have gone to mountain survival school as part of his military training, is hiding in a remote part of the mountainous and wooded state, making the search for him difficult.

He has been said to "pose a significant risk, if approached". Police have not provided a possible motive.

Wenatchee Police Department Man looks forward while wearing hatWenatchee Police Department

The girls - Paityn, Evelyn and Olivia - died from apparent suffocation in a remote campground earlier this week, according to police.

Their mother reported them missing on 30 May, after Mr Decker failed to return his daughters to her following a visit. He also did not take her phone calls.

A few days later, on 2 June, police found the girls' bodies near the campground.

Police also found Mr Decker's cell phone at the campground, but he was no longer there. His truck, which he was believed to have been living in, was also found nearby.

"I don't think any motive would be acceptable. And clearly it's not the decision of a sound mind," Chelan County Sheriff Mike Morrison said at a press conference on Wednesday, where he made a public plea to Mr Decker to turn himself in.

Mr Decker's outdoor survival skills may be helping him evade the authorities searching for him, which includes the FBI, Sheriff Morrison said.

"It sounds like at times he would go out and would be off-grid for sometimes up to two and a half months," Sheriff Morrison said after speaking to Mr Decker's family.

Mr Decker may have scoped out the area and hidden supplies there before allegedly killing his children, and he has "the knowledge to survive for a long period of time", the sheriff added.

"We want a peaceful resolution to this, but we're not going to relinquish our efforts," Sheriff Morrison said while asking Mr Decker to come forward. "Let's wrap this up and do what's right for your kids."

The US Marshals Service on Wednesday announced a $20,000 (?14,780) reward for information leading to Mr Decker's capture.

The search has caused widespread closures of national forests popular among hikers in the area. (BBC)

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