WASHINGTON- Israel did not ask for US backing in advance of a ground offensive into Gaza, US Vice President Dick Cheney said Sunday, as tens of thousands of Israeli troops backed by tanks pounded the Palestinian enclave.

"They didn't seek clearance or approval from us," Cheney told CBS television, adding that the Israelis, America's closest Middle East allies, "haven't told me exactly what they planned to do or when they plan to do it."

The US vice president made his remarks as Israel battled Hamas fighters in Gaza fields and roads, and as the Palestinian death toll from the offensive to end militant rocket attacks surpassed 500.

Israel long has maintained that "they didn't want to have to act ... but if the rocketing didn't stop, they felt they had no choice but to take action," the US vice president told CBS, speaking about Israel's biggest military operation since its 2006 war in Lebanon.

"The enemy is not the Palestinians, from the perspective of the Israelis -- it's Hamas," Cheney added, reiterating the US view that Israel has the right use massive force to defend itself against Hamas.

"To go after that terrorist organization, I think, they probably decided that an air campaign wasn't enough, that they had to go in on the ground, if they were going to take down the sites from which the rockets have been launched against Israel," he said.

Cheney also said Washington officials were "concerned" about the plight of the Palestinians. "They are the victims, in a sense, of Hamas, as well," he said.

But he said any cessation in hostilities would have to be "sustainable" and "durable."

"If there's to be a cease-fire, you can't simply go back to the status quo ante, what it was a few weeks ago, where you had a ceasefire recognized by one side but not adhered to by the other," Cheney said.

"It has to be a sustainable, durable proposition. Hamas has to stop rocketing Israel. And I don't think you're going to have a viable cease-fire until untill they're prepared to do that."