Israeli strikes killed more than 30 Gazans on Thursday, and another 11 died in three separate strikes on Friday, hiking the overall death toll to 103, Palestinian medics said. More than 500 people have been injured.

Overnight, the army said it hit 50 targets, also bombing 21 Hamas-owned structures in the past 24 hours.

One of the strikes hit the port in Gaza City, setting boats alight, including Gaza's Ark, a Palestinian-built protest vessel set to run Israel's naval blockade, an AFP correspondent said.

Israel has confirmed preparations are under way for a possible ground attack, with tanks and artillery massed along the border and some 33,000 reserves mobilized out of the 40,000 approved by the cabinet.

As the battle between Israel and Hamas entered its fourth day, diplomatic efforts to end the hostilities gathered pace.

Overnight, US President Barack Obama phoned Israeli Prime Minister Benjamin Netanyahu to express concerns and to offer his help in resolving the crisis.

"The United States remains prepared to facilitate a cessation of hostilities, including a return to the November 2012 ceasefire agreement," the White House said, referring to a truce which ended the last major confrontation between Israel and Hamas.

But Egypt, which has played a key role in mediating previous ceasefire agreements, said its efforts were falling on deaf ears.

"Egypt has communicated with all sides to halt violence against civilians and called on them to continue with the truce agreement signed in November 2012," the foreign ministry said.

"Unfortunately, these efforts... have met with stubbornness."

Neither of the warring sides appeared to have any interest in backing down.

NJF/NJF