Peres made the remarks while meeting with US President Barack Obama at the White House. Peres is in the US on an official visit, Xinhua reports.
"The best thing that could have happened was that Iraq remain a united country," Peres told Obama, according to the news website The Hill. "But I wonder if it's possible."
"To do so, you need (to be) ready to stand a mighty army to force all the three parties together. I don't see the army to do it and I don't see the parties that will agree to it," Peres said.
Obama and Peres also met with American Jewish community leaders, had a working lunch, in addition to having brief talks in the Oval Office Wednesday.
Peres will finish his final term as president next month after a 65-year political career. US Congress is expected to present him with the Congressional Gold Medal Thursday.