GAZA CITY (AFP) - Israel pounded Gaza for a fifth day on Saturday as it vowed no let-up in its air campaign to halt rocket attacks by militants which has killed more than 120 Palestinians.

A defiant Hamas fired five more rockets into Israel as the Islamist movement rejected growing international calls for a halt to hostilities, insisting Israel must act first.

Diplomatic efforts to stop the violence saw US President Barack Obama telephoning Israeli Prime Minister Benjamin Netanyahu and Washington offering to use its relationships in the Middle East to bring about 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 and preparations gathered pace for a possible ground assault.

"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."

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."

After weeks of rocket fire into its southern flank, Israel appeared bent on dealing a fatal blow to Hamas. 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 attack, 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. "I imagine we shall decide tomorrow (Saturday) or the day after on the next stage."

Early Saturday, 16 Palestinians died in a wave of Israeli air strikes as Operation Protective Edge entered its fifth day, taking the overall death toll to 121, medics said. The Israeli military said it had struck "several terrorists conspiring to launch rockets at Israel" and "a weapons cache concealed within a mosque in the central Gaza Strip."

So far, no one in Israel has been killed. Two have been seriously wounded, including a man at a petrol station hit by a rocket.

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," said White House spokesman Josh Earnest 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's human rights office over the number of civilian casualties. "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.

A group of 34 charities and NGOs also called for an end to the fighting. "Military actions by all parties must stop," said a statement signed by groups including ActionAid, CARE, Oxfam and Save the Children.

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 525 rockets have struck the Jewish state, and Iron Dome has shot down around 138, an army spokesman told AFP on Saturday.

The army Twitter account said that overnight strikes "hit over 60 terror targets, making a total of 1,160 since the beginning of Operation Protective Edge."