In a flurry of telephone diplomacy, Obama also commended Egypt's efforts to de-escalate the crisis during a call to President Mohamed Morsi, the White House announced after Friday's visit to Gaza by Egypt's Prime Minister Hisham Kandil.
The White House also noted that Israeli Prime Minister Benjamin Netanyahu had thanked the US for investing in Israel's new "Iron Dome" system which had intercepted "hundreds of incoming rockets from Gaza."
United Nations officials said chief Ban Ki-moon would travel to the Middle East to push for a truce.
Visiting Gaza on Saturday, Tunisian Foreign Minister Rafik Abdesslem condemned Israel's air campaign and urged the Arab League to "stop this flagrant aggression" on what he termed "our people in Gaza."
"What Israel is doing is not legitimate and is not acceptable at all," Abdesslem said.
Air raids persistent
Medics say three Palestinians were killed during a string of Israeli aid raids from Friday night into Saturday, raising the death toll in Gaza to at least 40 as the conflagration entered its fourth day.
Israel's toll stands at three killed from a rocket attack Thursday on a southern town. Friday's rocket firings toward Jerusalem and Tel Aviv caused no casualties but triggered panic in Israel's main population centers, said Israeli police.
Hamas, the Islamist group that runs Gaza, said Israeli warplanes had bombed the office building of its leader, Ismail Haniyeh - where he had just met Kandil on Friday - as well as a police headquarters.
Israel's Channel 2 television said the Cabinet had approved the call-up of as many as 75,000 army reservists. Israel's military also announced the closure of highways leading to Gaza, prompting further speculation that Israel might invade the enclave of 1.7 million Palestinians as it did around New Year 2009.
Israel Radio's military affairs correspondent said the army had told municipal officials that fighting could drag on for seven weeks. An Israeli military spokeswoman declined to comment on that report.
Israel's military said a rocket fired early on Saturday by Gaza militants had "lightly injured" four soldiers in the Eshkol region, which lies along the southern flank of the border with Gaza.
ipj/sej (dpa, AFP, Reuters)