A US court has fined an 85-year-old former army engineer $50,000 after he admitted passing classified documents to Israel.

Ben-Ami Kadish apologised on Friday, after pleading guilty in December to acting as an unregistered agent of Israel during the 1970s and 80s.

"It was a mistake. It was a misjudgment," he said during sentencing at the court in New York.

"I thought I was helping the state of Israel without harming the United States," he said.

Prosecutors had recommended Kadish be spared a prison sentence.

But US District Judge William Pauley raised questions over why the authorities had waited so long to bring charges.

"Why it took the government 23 years to charge Mr. Kadish is shrouded in mystery," Pauley said.

"It is clear the (US) government could have charged Mr. Kadish with far more serious crimes."

Kadish reported to Israeli agent Yosef Yagur, who has been linked in court documents to the case of Jonathan Pollard, a US citizen who is serving a life sentence for a 1985 charge of spying for Israel.