Gaza is braced for a ground invasion by Israeli forces following intensified bombing that included the flattening of the headquarters of the Hamas leader Ismail Haniyeh.

As the Israeli military began the emergency call-up of up to 75,000 reservists, leaders from Turkey, Egypt and Qatar met in Cairo to discuss ways of ending the escalating violence. Since Wednesday Israel has launched about 1,000 air strikes on the coastal Palestinian territory.

The raids have continued past midnight into Sunday. Two pre-dawn attacks on houses in the Jabiliya refugee camp killed two children and wounded 13 other people, Gaza medical officials said.

Witnesses said a building in Gaza City housing the offices of Hamas's al-Aqsa TV was bombed, wounding six journalists. The building was also used by Arabic and international news outlets including Germany's ARD, Kuwait TV, the Italian RAI, Sky News of Britain and others.

The Israeli military also appeared to take over the frequencies of the radio stations of Hamas and the Islamic Jihad militant group to broadcast a recorded, Arabic-language warning to the people of Gaza to stay away from Hamas installations and personnel. "To the people of Gaza, Hamas is playing with fire and gambling with your fate. The Israel defence force is moving toward the second phase of its operation. For your safety you should stay away from Hamas infrastructure and personnel."

The message did not say what the "second phase" was but thousands of Israeli troops are massed near the Gaza border awaiting an order to invade.

Overnight, aircraft targeted dozens of underground rocket launchers and a major Hamas training base and command centre, the Israeli military said. It said it also attacked a communications antenna and gunboats fired on militant sites on the Gaza shoreline.

In all 48 Palestinians, about half of them civilians, have been killed and more than 400 civilians wounded, according to Gaza medical officials. The World Health Organisation said Gaza hospitals were overwhelmed with casualties and their medical supplies were running short.

Three Israeli civilians have been killed and more than 50 wounded.

The US urged diplomacy and "de-escalation" but said Israel had the right to self-defence. It wanted the "same thing as the Israelis want" in ending rocket attacks, the White House said in a statement. In Tel Aviv air raid sirens sounded for the third day running, with residents reporting the sound of an explosion. Hamas said it had fired a Fajr-5 rocket from its arsenal of long-range missiles. Three Israelis have been killed since the conflict began on Wednesday.

A small mountain of rubble, twisted metal and broken glass was all that remained of Haniyeh's headquarters. Several Palestinian flags fluttered on poles poking out from the debris. Strikes from Israeli aircraft and warships continued to pummel the Gaza Strip throughout Saturday.

Israel's hardline foreign minister, Avigdor Lieberman, warned of a "crushing response" still to come to prevent missile fire by Hamas and other militant groups but denied that Israel had launched an all-out war.

"The only way we can achieve peace and security is to create real deterrence via a crushing response that will make sure they don't try to test us again," he said. "This isn't an all-out war but an operation with defined goals." If a ground invasion were authorised Israel would have to "see it through," he said. "This wasn't done during Operation Cast Lead [the 22-day war four years ago], which is why we failed to achieve our goal."

On a visit to Gaza the Tunisian foreign minister, Rafik Abdesslem, denounced the Israeli attacks as unacceptable and against international law. "Israel should understand that many things have changed and that lots of water has run in the Arab river," he said. "It should realise it no longer has a free hand. It does not have total immunity and is not above international law … What Israel is doing is not legitimate and is not acceptable at all."

He was expected to later join a meeting in Cairo of regional leaders, along with Hamas's Khaled Mashaal and Ramadan Shallah, the Islamic Jihad secretary general, to discuss ways of trying to contain the crisis. Others at the gathering included Egyptian President Mohamed Morsi, Turkey's prime minister, Recep Tayyip Erdogan, and the Qatari emir.

The meeting came as Arab League foreign ministers also met to draw up a draft statement calling for a negotiated ceasefire. The league was further expected to authorise its general secretary, Nabil Elaraby, to lead a delegation into Gaza following visits in recent days.

Erdogan has been a highly outspoken critic of Israel, while Qatar's emir, Sheikh Hamad bin Khalifa al-Thani, visited Gaza in October, breaking the isolation of the Hamas government.

Arab League diplomats briefed on Saturday evening that its statement would be calling for an immediate ceasefire. However Hamas officials in Gaza said that any truce would be dependent on Israel agreeing to lift its long-term blockade of the territory and agree to end its policy of assassinations of Hamas leaders, conditions that Israel is unlikely to accept.

The disclosure of the terms of the talks began to emerge as Hamas fired a Fajr-5 missile at the outskirts of Tel Aviv, which is likely to reinforce Israel's willingness to push ahead with the campaign after a day of continuing rocket fire out of Gaza and Israeli air strikes.

According to military sources, Egyptian intelligence officials met Hamas officials in Gaza for the second day running. The talks coincided with recent high-profile delegations that have entered Gaza to show solidarity with Palestinians living there.

Tunisian foreign minister Rafik Abdesslem visited Gaza a day after Egyptian prime minister Hisham Kandil crossed the border into the Palestinian enclave, condemning Israeli actions and pledging to work for a truce. Meshaal also held talks on Saturday with Egyptian security officials on prospects for a truce.

Egypt brokered an informal truce in October, which has since collapsed. It now says it is seeking a new deal.

An Arab diplomatic source, who declined to be named, told Reuters the Arab League draft to be discussed by the ministers expressed the Cairo-based league's support for Egypt's efforts to achieve a "long-term truce" between Israel and Palestinian factions.

The draft also calls for the UN security council to take the necessary steps to halt the violence and "protect the Palestinian people".

Rumours emerged late on Saturday that possible truce talks could take place in Cairo as early as Sunday.