A 71-year-old Norwegian man was sentenced to 12.5 years in prison on Monday for ordering sexual assaults to be carried out online against dozens of young Filipinas.

"With his actions, the accused contributed to a sex industry that brutally exploits vulnerable children in the worst possible way," the Sore Sunnmore district court in western Norway wrote in its verdict.

Between 2012 and 2013, the man used an online video calling service to contact people in the Philippines.

He paid them to, under his instructions, sexually assault minors under the age of 14, the youngest one "probably between three and five years old", while he watched, the court found.

The court said there were "at least" 48 victims but the actual number could be much higher because of difficulties identifying them.

"The court finds that the victims are living in an almost inhumane situation that the accused helped perpetuate with his actions," the verdict said.

Norwegian police were alerted by the United States FBI as part of a larger case the latter was probing.

Investigators found more than 36,000 photographs and videos containing child pornography in his possession.

"I went into a bubble and I was going through a very difficult time in my personal life," the accused told the court during his trial, according to broadcaster NRK.

The sentence handed down was slightly higher than the 12 years the prosecution had called for.

The victims' young age was considered an aggravating factor for the sentencing.