Cohen called the nuclear deal a "terrible mistake," saying it allows Iran to keep key elements of its nuclear program intact and will remove other restraints in a few years.

"Then Iran will be able to enrich enough uranium for an arsenal of nuclear bombs," Cohen said, according to the meeting participant, who spoke on condition of anonymity because he was discussing a classified security matter.

Cohen also criticized the deal’s easing of sanctions, saying it has resulted in "significantly increased" aggression by Iran, and noted Iran’s continued development of long-range ballistic missiles.

"As head of the Mossad, I am 100 percent certain that Iran has never abandoned its military nuclear vision for a single instant. This deal enables Iran to achieve that vision," Cohen said. "That is why I believe the deal must be completely changed or scrapped. The failure to do so would be a grave threat to Israel’s security."