The White House declared Thursday that it was up to the Islamist Palestinian Hamas movement to "de-escalate" a deadly conflict with Israel by halting its "cowardly" rocket attacks at the staunch U.S. ally.

"We strongly condemn the barrage of rocket fire from Gaza into Israel, and we regret the death and injury of innocent Israeli and Palestinian civilians caused by the ensuing violence," press secretary Jay Carney told reporters aboard Air Force One.

"There is no justification for the violence that Hamas and other terrorist organizations are employing against the people of Israel," Carney said. "We call on those responsible to stop these cowardly acts immediately in order to allow the situation to de-escalate."

Carney's comments came as Reuters reported that a rocket fired by Hamas killed three Israelis, even as the Palestinian death toll rose to 15 amid concerns of an all-out war between America's closest ally in the Middle East and the Islamist government of the Gaza Strip. (The Israeli military has been practically live-tweeting its actions and has set up a Flickr feed for its operations.)President Barack Obama spoke by telephone late Wednesday with Israeli Prime Minister Benjamin Netanyahu and, separately, with Egyptian President Mohamed Morsi.

"In both conversations, the president reiterated the United States' support for Israel's right to self-defense. President Obama also urged Prime Minister Netanyahu to make every effort to avoid civilian casualties," Carney said.

"Hamas claims to have the best interests of the Palestinian people at heart, yet it continues to engage in violence that is counterproductive to the Palestinian cause," Carney said. "Attacking Israel on a near-daily basis does nothing to help the Palestinians in Gaza or to move the Palestinian people any closer to achieving self-determination."