US President Barack Obama on Thursday told Israeli Prime Minister Benjamin Netanyahu he was worried fierce fighting with Hamas in Gaza could escalate, and offered US help to broker a ceasefire.

Obama spoke to the Israeli leader after Israeli warplanes pounded the Palestinian enclave but did not stop militants firing rockets at cities inside the Jewish state, and as global concern over rising Palestinian casualties mounted.

"The president expressed concern about the risk of further escalation and emphasized the need for all sides to do everything they can to protect the lives of civilians and restore calm," the White House said in a statement.

"The United States remains prepared to facilitate a cessation of hostilities, including a return to the November 2012 ceasefire agreement."

The 2012 deal, brokered by former secretary of state Hillary Clinton and Egypt, ended eight days of Israeli airstrikes on Hamas targets in a previous showdown.

The White House statement said that Obama told Netanyahu that he understood Israel had a right to defend itself and that he condemned rocket attacks by Hamas and other militant groups launched from Gaza.

Obama expressed sympathy for the deaths of three Israeli teenagers in the West Bank that triggered new tensions between the Jewish state and Palestinians and praised Israel for swiftly making arrests in the case of a Palestinian youth apparently killed in reprisal for the incident.

Obama also expressed concern over the case of a Palestinian-American teenager allegedly beaten in police custody.

The two leaders also discussed the effort to conclude a deal on curtailing Iran's nuclear program by an approaching July 20 deadline. Netanyahu has previously expressed deep skepticism of the US approach toward Iran. (AFP)