The major concern of the United States in the current Israeli-Hamas conflict is a potential Israeli ground incursion into Gaza, U.S. officials said Friday.

That would be a disastrous escalation that could trigger a larger conflict, a senior U.S. official told CNN.

"Escalation is what we are concerned about. We don't want it to escalate to the point where Israel feels it has to take additional action, specifically ground force action," the official said.

Israel has some ground forces on the border and ready to act if they are called to do so, said Israel's ambassador to the United States, Michael Oren, in a conference call with reporters Friday.

Israel's Cabinet on Friday authorized the call-ups of up to 75,000 reservists, prime minister's spokesman Mark Regev said. Israel's military said earlier in the day that 30,000 troops are being mobilized for a possible move into Gaza.

So far, Israeli tank units and troops have stayed outside Gaza. But the senior U.S. official indicated while the United States unequivocally supports Israel's right to self-defense, the U.S. message right now is for Israel not to enter Gaza on the ground.

"Ground forces would clearly be a major escalation," he said.

The official said the worry is Israeli ground troops could lead to drawing Egypt into the conflict.

"What action would Egypt take, would they move into Sinai?" he said.

He said the assessment is that a number of other Arab nations in the region would then, at least rhetorically, join with Egypt against Israel. The official emphasized this is a U.S. worry and the strong hope is Hamas will stop its rocket attacks on Israel, thereby ending Israeli strikes.

"The question also is how much Hamas wants to risk," the official said.

A second official said the United States also is watching closely to see if Iran, which backs Hamas, attempts to infiltrate new fighters or weapons supplies into Gaza. Neither official would speak on the record because they were discussing sensitive national security matters.

Daniel Ben Simon, an Israeli Knesset member not aligned with Prime Minister Benjamin Netanyahu's coalition, said Friday that he sensed "almost an absence" from Washington, adding, "It's time for the Americans to step in."

"We need the boss, and the boss is still the White House," Ben Simon said, calling U.S. officials "the only ones that can control the situation and ... talk freely" to all sides. "It seems the White House is too silent."

U.S. President Barack Obama did talk Friday with Netanyahu, after the Israeli prime minister called him to voice "deep appreciation ... for the United States' investment in the Iron Dome rocket and mortar defense system," the White House said.

"The president reiterated U.S. support for Israel's right to defend itself, and expressed regret over the loss of Israeli and Palestinian civilian lives," the statement said. "The two leaders discussed options for de-escalating the situation."

Obama called Egyptian President Mohammed Morsy, commending "Egypt's efforts to de-escalate the situation and (expressing) his hope that these efforts would be successful," the White House said in another statement. The president also "expressed regret for the loss of Israeli and Palestinian civilian lives."

The State Department says Secretary of State Hillary Clinton has made a number of phone calls trying to muster international pressure to stop what it terms a "very, very dangerous situation" in Gaza.

Clinton, traveling in Asia, called Israeli Foreign Minister Avigdor Lieberman and Egyptian Foreign Minister Mohamed Kamel Amr for the second time in as many days, as well as Jordanian King Abdullah and Turkish Foreign Minister Ahmet Davutoglu, State Department spokeswoman Victoria Nuland said Friday.

"In all cases her message has been the same: that we are urging a de-escalation of this conflict, we are urging those countries with influence on Hamas and other groups in Gaza to use that influence to get a de-escalation."