Israel pounded Gaza for a fifth day Saturday with air strikes and artillery, killing 22 Palestinians as Hamas defiantly kept up its rocket fire into the Jewish state.

Both sides brushed off international calls for a truce and Israel kept up its buildup of troops and armour on the Gaza border in preparation for a possible ground invasion.

Saturday's strikes, which Gaza emergency services said hit a mosque and a centre for the disabled among other targets, brought the death toll since the Tuesday start of Israel's Operation Protective Edge to 127.

The Israeli army said the mosque housed an arms cache.

It said that over the same period 530 rockets hit Israel, nine of them on Saturday.

US President Barack Obama called Israeli Prime Minister Benjamin Netanyahu on Friday and Washington offered to use its influence in the Middle East to bring a return to calm.

But speaking at a news conference in Tel Aviv on Friday, Netanyahu said he would not end the military campaign until he achieved his goal of stopping the Hamas fire.

"No international pressure will prevent us from striking, with all force, against the terrorist organisation which calls for our destruction," he said.

"No terrorist target in Gaza is immune."

The latest border flareup -- the deadliest since November 2012 -- can be traced to last month's kidnap and murder of three young Israelis in the occupied West Bank and the brutal revenge killing of a Palestinian teenager by Jewish extremists.

Israel responded with a major crackdown on Hamas, even though the Islamist group declined to confirm or deny its involvement, while Gaza militants hit back with intensified rocket fire.

Despite international concern, truce efforts have been unsuccessful, according to Egypt, which has been key in mediating previous ceasefires between Hamas and Israel.

"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."

Former British premier Tony Blair, the envoy for the so-called Quartet of Middle East diplomatic players, flew into Cairo on Saturday for talks on ending the violence.

Ismail Haniya, Gaza's former premier and the most senior Hamas official in the coastal enclave, ruled out any halt to hostilities.

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

- Ground attack decision soon -

Israel says preparations are under way for a possible ground incursion, with tanks and artillery massed along the border and some 33,000 reservists mobilised out of 40,000 approved by the cabinet.

More armour was seen heading south on Saturday morning.

Foreign Minister Avigdor Lieberman said he expected a political decision on a possible ground operation by Sunday.

"At the moment we are dealing with the first phase... air attacks," he told Channel One television on Friday.

"I imagine we shall decide tomorrow (Saturday) or the day after on the next stage."

So far, no one in Israel has been killed. Two have been seriously wounded.

In northern Israel, at least one rocket fired from Lebanon struck an open area near the town of Metula on Friday, prompting troops to respond with shelling, the army said.

The military believed a Palestinian group fired it in solidarity with Hamas, public radio reported.

- 'Military actions must stop' -

The escalating violence brought more offers of truce negotiations from the White House Friday.

"There are a number of relationships the United States has that we are willing to leverage in the region to try to bring about an end to the rocket fire that's originating in Gaza and, as we saw this morning, in Lebanon," White House spokesman Josh Earnest said on Friday.

He referred to taking steps as the US and Egypt did in November 2012 to broker a truce between Israel and Hamas.

Kuwait requested an emergency Arab foreign ministers meeting to discuss "the deteriorating situation", which a diplomat at the Arab League said will be held on Monday.

Israeli strikes on residential buildings in Gaza brought a rebuke from the UN human rights office over civilian casualty toll.

"Even when a home is identified as being used for military purposes, any attack must be proportionate... and precautions must be taken to protect civilians," said spokeswoman Ravina Shamdasani.

Amnesty International called for the United Nations "to immediately impose a comprehensive arms embargo on Israel, Hamas and Palestinian armed groups" and launch an enquiry into "violations committed on all sides".

Since the start of Israel's operation on Tuesday, about 530 rockets have struck the Jewish state, and the Iron Dome air defence system has shot down around 138, an army spokeswoman told AFP on Saturday.