Carter also described the siege on Gaza as a crime and an atrocity on Thursday.
Carter said Palestinians in Gaza were being "starved to death", receiving fewer calories a day than people in the poorest parts of Africa.
"It's an atrocity what is being perpetrated as punishment on the people in Gaza," he said. "It's a crime... I think it is an abomination that this continues to go on."
Carter also said that US attempts to undermine Hamas have been counterproductive.
His meeting with leaders of Hamas, a group he insists must be included in peace negotiations, has angered many in Washington and Israel, who call the Palestinian group a terrorist organisation.
Israel has meanwhile sealed the West Bank and Gaza for 10 days as it prepares to celebrate the Jewish holiday of Passover.
Tanks and soldiers moved into position at the crossings following the shutdown that took effect early on Friday barring Palestinians from the occupied territories from entering Israel.
The shutdown follows a statement by Hamas on Thursday that "all option are open" to avenge the killing of 20 Palestinians in a day of air assaults and ground battles in Gaza that also left three Israeli soldiers dead.
In a statement published on the internet, the armed wing of Hamas called on its fighters to attack Israel "in every place and with all means available".
"This enemy only understands the language of force," it said.
The sealing of the borders also comes after the Israeli military said on Thursday that it had foiled a third attempt in the past week by Palestinian fighters to infiltrate the Gaza crossing.
The army said three Palestinian fighters attacked Kerem Shalom, a crossing used to deliver humanitarian supplies to Gaza.
One attacker was killed while a second was wounded when Israeli troops opened fire. The third man escaped, the military said.
The Israeli military said troops would be on high alert during the holiday period, regarding it "as a highly sensitive time, security wise".
It said it would try its best to preserve the daily life of Palestinians, allowing humanitarian cases, doctors and lawyers to cross the border.
But Israel has already been restricting the flow of food and fuel supplies into Gaza since Hamas seized control of the area last June.
Israel says the restrictions are in response to repeated rocket attacks launched from there and they have been tightened in recent weeks in response to continued heavy fighting.