"I don't believe there is any possibility to have peace between Palestinians and Israel unless Hamas is involved directly in harmony with Fatah," he said, according to a Reuters report.
Carter's remarks come amid aggressive diplomatic efforts by the Obama administration for a peace deal through a two-state solution. Secretary of State Clinton has made it clear that the U.S. and European-designated terror group will not be part of peace process unless it renounces violence and recognize Israel.
Israeli Prime Minister Netanyahu has also refused to talk to Hamas.
Carter said he hoped to see a prisoner exchange agreement between Hamas and Israel involving Corporal Gilad Shalit and Palestinian prisoners, including Marwan Barghouti.
Shalit, an Israeli soldier, has been in Hamas custody since his abduction in 2006, although the group has not demonstrated proof of life and will not let the International Red Cross visit him. Israel currently holds nearly 11,000 Palestinian prisoners, including Barghouti, who is currently serving five life sentences for his involvement in suicide attacks in Israel.
Prisoner releases have taken place in the past between Israel and the Palestinians, including more than 450 Palestinians released last year.
Carter will travel to Israel after his visit to Syria. He is expected to visit with Israeli officials.