"A ground invasion is much more difficult for the international community to sympathize with or support, including the United Kingdom," he said.
Israeli officials say the airstrikes are aimed at ending months of rocket fire out of the Hamas-ruled territory.
Israel began the offensive with an unexpected airstrike that killed Hamas' military chief, and since then has targeted suspected rocket launchers and storage sites.
The Mideast ally is now at a crossroads: launch a ground invasion or pursue Egyptian-led truce efforts.
But with Israel and Hamas far apart on any terms of cease-fire, Israeli Prime Minister Benjamin Netanyahu declared at the start of the weekly Cabinet meeting, "The Israeli military is prepared to significantly expand the operation."
The crisis threatened to overshadow Obama's trip to Asia, which includes stops in Myanmar and Cambodia as part of a broader effort to expand the U.S. economic and military presence in a region long dominated by China.
So far, the U.S. has thrown its weight behind Israel, and Obama has called on Egypt and Turkey to intervene on Israel's behalf.
