Rocket attacks fired on Israel by Palestinian militants in Gaza were a "precipitating factor" in the conflict that has engulfed the two sworn enemies, a top US official said Saturday.

Deputy National Security Adviser Ben Rhodes said both President Barack Obama and Israeli Prime Minister Benjamin Netanyahu agree that de-escalation is preferable, provided that Hamas ceases firing on Israel.

The two leaders have spoken by telephone every day since the situation unfolded, he said.

"We believe that the precipitating factor for the conflict was the rocket fire coming out of Gaza," Rhodes told reporters aboard Air Force One as Obama headed to Asia.

"We believe that Israel has a right to defend itself, and they'll make their own decisions about the tactics that they use in that regard."

His comments came as the Jewish state called up thousands more reservists, edging closer to launching its first ground offensive on the tiny Palestinian coastal enclave in four years.

Obama also spoke by telephone with Egyptian President Mohamed Morsi and Turkey's Prime Minister Tayyip Erdogan. Washington has urged both leaders to press Hamas to stop firing rockets into Israel.

"They have the ability to play a constructive role in engaging Hamas and encouraging a process of de-escalation," Rhodes said.

As he spoke, the Turkish leader was in Egypt for a visit overshadowed by Israel's aerial assault on neighboring Gaza.

"The Israelis are going to make decisions about their own military tactics and operations," Rhodes said. "What we want is same thing the Israelis want, which is an end to the rocket fire coming out of Gaza."

The senior aide disputed that an Israeli airstrike that killed top Hamas military commander Ahmed Jaabari on Wednesday had triggered the fresh outbreak of violence.

"Just to be clear on the precipitating factor, these rockets have been fired into Israeli civilian areas and territory for some time now. So Israelis have endured far too much of a threat from these rockets for far too long, and that is what led the Israelis to take the action that they did in Gaza," he said.

Asked about Israeli targeting of government buildings, Rhodes said that "we wouldn't comment on specific targeting choices by the Israelis other than to say that we, of course, always underscore the importance of avoiding civilian casualties.

"But the Israelis, again, will make judgments about their military operations," he added.

Israeli strikes on Gaza destroyed the Hamas government headquarters earlier, including the office of Hamas prime minister Ismail Haniya.

Israeli Deputy Foreign Minister Danny Ayalon said an Israeli ground invasion could be avoided if Palestinian militants stop firing rockets on Israel.

"It all depends, the next 24 hours or so, if we will see peace and quiet from Hamas," Ayalon told CNN.

If they "stop the fire at us, I think we can still avoid it," he said.

Medics say 40 Gazans have been killed and more than 390 wounded since Israel launched its aerial campaign this week, with at least five militants among 10 Palestinians killed on Saturday.

Since the start of Operation Pillar of Defense, three Israelis have been killed and 18 injured, including 10 soldiers. The Israeli army said its air force had hit more than 950 targets in Gaza.