Waltz said on X that he was in Jordanian capital Amman to discuss speeding up the delivery of humanitarian aid to Gaza and "to see the vital work Jordanian and UN partners are doing".
"We're ready to move forward with the rebuilding of Gaza... So we're here to knock down those bureaucratic barriers and get things moving," he added.
Kallas was also in Amman on Sunday, where she held talks with King Abdullah II on the Gaza ceasefire and regional developments, the royal palace said in a statement.
Abdullah notably reiterated "the need to adhere to the terms of the agreement to end the war on Gaza and ensure the flow of relief aid", the palace said.
Kallas and the king discussed "the need to achieve comprehensive calm in the region and preserve the sovereignty of states", it added.
Their visits came as Israeli Prime Minister Benjamin Netanyahu said he expected the second phase of the US-sponsored ceasefire plan for the Palestinian territory to begin soon.
Despite the ongoing truce, Israel has continued to massacre Palestinians in the besieged strip, killing 367 Palestinians, many of them women and children, since it came into effect on 10 October.
Since the outbreak of Israel's two-year genocidal war on Gaza, Israeli forces have killed over 70,000 Palestinians, most of them women and children, and wounded more than 171,000 others.
Vast areas of the enclave have been destroyed, with the United Nations (UN) estimating reconstruction costs at more than $70 billion.
The second stage of the Gaza ceasefire plan concerns disarming Hamas, establishing a transitional authority and deploying an international stabilisation force, paving the way for an Israeli withdrawal from the territory.
Hamas said Saturday it is ready to hand over its weapons in the Gaza Strip to a future Palestinian “state” that will govern the territory, but only if “the Israeli army’s occupation ends.”
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