Friends of a New Zealand man missing in the United States are still hopeful he will be found alive and well.

Father-of-two Peter O'Brien, 44, has been missing from his home in Boise, Idaho, since April 2, when his ex-wife Suzanne Oppenheimer last saw him.

Ms Oppenheimer said she expected Mr O'Brien to pick up his five and six-year-old sons on April 4 and went around to his place when he did not show up.

When she arrived at his apartment, she found the door unlocked and his wallet, car keys and cellphone inside. His car was parked in the drive.

Long-time friend Martin Lopez told NZPA today both ground and aerial search teams had failed to find any signs of Mr O'Brien in the areas surrounding his home but friends and family had not given up hope.

"I've known him for about 10 years and I think he was under a lot of pressure ... I think he'll make contact."

Mr Lopez said family had started to widen their search.

Police don't believe Mr O'Brien has left the country as his passport has not been used.

Detectives were now checking bank records and security footage at nearby airports to establish if he had travelled within the United States.

Ms Oppenheimer said she had no particular fears about the Mr O'Brien's mental state before he disappeared, but that their relationship was strained.

Mr O'Brien, who was originally from Wellington, had been living in the US since 2000 after making a move from Australia.

His brother and one of his sisters have flown to the US to help with the inquiry.

NZPA