Israel's bombardment of Gaza claimed its 100th Palestinian life on Friday as Hamas pounded central Israel with rockets, and Washington offered help brokering a truce.

As the battle between Israel and Hamas militants 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.

After weeks of rising rocket fire on its south, Israel lost patience and was bent on dealing a fatal blow to Hamas, with Netanyahu reportedly saying talk of a ceasefire was "not even on the agenda".

And Ismail Haniya, Gaza's former prime minister and the most senior Hamas official in the enclave, also ruled out any end to hostilities.

"(Israel) is the one that started this aggression and it must stop, because we are (simply) defending ourselves," he said.

Airport under fire

Hamas's armed wing, the Ezzedine al-Qassam Brigades, has kept up a steady stream of rockets deep inside Israel, with sirens sending people fleeing for shelter in Jerusalem, Tel Aviv and even in the northern port city of Haifa.

On Friday morning, three Gaza rockets were shot down over Tel Aviv by the Iron Dome anti-missile system, the army said, as the Brigades claimed their militants had fired M75 missiles at Israel's main international airport.

Israel's Airport Authority said Ben Gurion airport had been closed for "nine minutes" during the raid, but then normal operations were resumed.

But Hamas warned "all foreign airlines" to halt flights due to "the dangers surrounding all the airports due to the ongoing war".

It has also called for mass demonstrations across the region as Muslims mark the second Friday of the holy month of Ramadan. Although police prepared for possible disturbances, there was no immediate reports of any protests.

Israeli strikes killed more than 30 Gazans on Thursday, and another eight died in three separate strikes on Friday, hiking the overall death toll to 100, Palestinian medics said. More than 500 people have been injured.

So far, no-one in Israel has been killed, and only 10 people hurt, two of them seriously, medics said.

A 61-year-old man was very badly hurt Friday when a rocket struck a petrol station in the southern port of Ashdod, after a soldier was severely wounded in a mortar attack the night before, officials said.

Passenger jet diverted

At least one rocket fired from Lebanon struck northern Israel on Friday, prompting troops to hit back with artillery fire, the army said. The rocket hit an open area without causing any casualties or damage.

Military officials told public radio they believed a Palestinian group had fired in solidarity with Hamas, raising fears the violence in Gaza could spread to other fronts in a region already fraught with conflict.

As a result, a passenger jet heading to Tel Aviv was forced to make an emergency stop in Cyprus, before returning to Poland, aviation authorities said.

The Polish Lot aircraft "was instructed to wait in the air. The pilot decided to head to an alternative airport in Larnaca, and after waiting there, returned with the passengers to Poland."

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.

Since the start of the operation, 426 rockets have struck Israel, and Iron Dome has shot down 121. Another 32 crashed into Israel since midnight, with 11 more intercepted, the army 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 mobilised out of the 40,000 approved by the cabinet.