Mr Carter advocates talks with Hamas
Former US President Jimmy Carter has said he had to "hold back tears" while viewing destruction on a visit to Gaza.
He met leaders from Hamas, which controls Gaza but is shunned as a terrorist group by western countries.
The veteran politician handed over a letter from the family of captured Israeli soldier Gilad Shalit, who he said he believed was "alive and well".
He criticised Israel's blockade and January military operation in Gaza, and called for an end to all violence.
The former US president, who brokered the 1979 Israel-Egypt peace deal, has long advocated engagement with the militant Hamas movement as crucial for progress on peace.
Standing beside Ismail Haniyeh, a Hamas leader and former Palestinian prime minister, Mr Carter said he was visiting Gaza as a private citizen, but would be be passing on what he had seen and heard to President Barack Obama.
Mr Haniyeh praised a "new language and new spirit" coming out of Washington, and reiterated that Hamas would support "a genuine project" aimed establishing a Palestinian state "with full sovereignty" within the pre-1967 borders.
Although Hamas's charter calls for Israel's destruction, leaders have offered a 10-year truce in exchange for a state in the West Bank and Gaza, occupied by Israel in 1967.
A spokesman for Hamas's military wing told AP news agency it would "study the possibility" of delivering the letter from the family of Gilad Shalit, who has been in captivity for three years.
Mr Carter condemned "deliberate" destruction in Israel's January offensive, but also expressed sadness over Palestinian rocket attacks on Israeli towns.
Visiting the American School in Gaza, damaged in Israel's three-week operation, Mr Carter said "it's very distressing to me".
He said the school had been "deliberately destroyed by bombs from F-16s made in my country and delivered to the Israelis".
Mr Carter also criticised Israel's blockade of the Strip, which prevents all but humanitarian basics entering Gaza - and extends to a ban on virtually all building materials.
Gazans "are treated more like animals than human beings," Mr Carter said.
"Never before in history has a large community like this been savaged by bombs and missiles and then been deprived of the means to repair itself," he said.
Visiting Israel earlier this week, Mr Carter said a major policy speech given by Israeli Prime Minister Benjamin Netanyahu "raised many new obstacles to peace".
While Mr Netanyahu yielded to US pressure to back the creation of a Palestinian state, he set the conditions that it must be demilitarised and recognise Israel as a Jewish state.