Israel's Foreign Minister Avigdor Lieberman is promising to inflict harsher military action after Palestinian militants fired a rocket aimed at Jerusalem Friday, just hours after a rocket targeted Tel Aviv.

Lieberman said options included the possible assassination of Hamas' prime minister, Ismail Haniyeh, and other top leaders.

"Every time that Hamas fires there will be a more and more severe response," he told Channel 2 TV late Friday. "I really recommend all the Hamas leadership in Gaza not to try us again. ... Nobody is immune there, not Haniyeh and not anybody else.".

UN Secretary General Ban-ki Moon is expected to visit the region within days. UN spokesman Martin Nesirky said Friday the UN chief will go to the region "shortly" to "push for an end to violence."

The attack marks the first time the holy city has ever been targeted by rockets fired by Gaza militants, a major escalation that could draw the possibility of an Israeli ground invasion closer. Witnesses said they saw a stream of smoke in Mevasseret Zion, a Jerusalem suburb.

Israeli police spokesman Micky Rosenfeld said the rocket landed in an open area near Gush Ezion, a collection of Jewish settlements in the West Bank southeast of the city.

Abu Obeida, spokesman for the Hamas militant wing, said the group had fired a long-range rocket at Jerusalem.

"We are sending a short and simple message: There is no security for any Zionist on any single inch of Palestine and we plan more surprises," he said. Hamas officials said the rocket was a homemade "M-75" rocket, a weapon that has never been fired before.

Before dawn on Saturday, missiles smashed into a small Hamas security facility as well as the sprawling Hamas police headquarters in Gaza City, setting off a massive blaze there that threatened to engulf nearby houses and civilian cars parked outside. No one was inside the buildings at the time.

A separate airstrike leveled a mosque in central Gaza, damaging nearby houses, Gaza security officials and residents said.

The Israeli military offensive has been limited to airstrikes so far, but the military has called up 16,000 reservists ahead of possible Gaza invasion. Troops were seen massing near the border. The government has also increased the cap for reservists from 30,000 to 75,000.

An earlier rocket attack aimed at Tel Aviv also marked the second straight day that Gaza militants have targeted Israel's largest city and commercial capital. The rocket landed in an open area near Tel Aviv, and no injuries were reported, said the CBC's Derek Stoffel.

Tel Aviv has opened its public bomb shelters, and hospitals are on increased alert, Stoffel added.

Israel had offered earlier to hold back its offensive in the Gaza Strip during a brief visit Friday by Egypt's prime minister Hesham Kandil.

Amid fresh rockets from Gaza, Israel resumed its airstrikes and smoke could be seen rising from over Gaza on Friday morning, said Stoffel, reporting from the city of Sderot near the Gaza border.

In three days of fierce fighting, at least 27 Palestinians, including 14 militants and six children, as well as three Israelis have been killed.

On Friday, Prime Minister Stephen Harper expressed his support for Israel, but urged both sides to "spare any innocent lives".

"We have been concerned, obviously, for some time about the presence of a terrorist group, Hamas, in charge in the Gaza Strip," he told reporters Friday.

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"We condemn this terrorist group's attacks on Israel. We recognize and support Israel's right to defend itself against such terrorist attacks. But obviously, we urge all sides to take all precautions possible to spare any innocent lives."

Israel began the offensive Wednesday by assassinating Hamas's military chief and striking dozens of rocket launchers. But militants have continued to rain rockets across Israel.

Brief lull in violence

An official in Prime Minister Benjamin Netanyahu's office said the Israeli leader's offer of a pause in its attacks — contingent on militants refraining from firing rockets at Israel — was in response to an Egyptian request.

Israel said it halted its incessant air attacks on militant targets in Gaza during the brief visit, though Hamas security claimed three airstrikes hit the territory during that period.

More than 50 rockets were fired from Gaza into southern Israel since sunrise, the CBC's Stoffel reported.

A Palestinian woman shouts in front of a house damaged in an Israeli airstrike Friday. (Suhaib Salem/Reuters)

Most of the rockets do not have guided systems, limiting their accuracy, though Israeli officials believe the militants may have a small number of guided missiles that have not yet been deployed.

Hamas said the two rockets aimed at the two Israeli cities Friday were made in Gaza, a prototype the militants call M-75, and have a range of about 80 kilometres.

Egyptian Prime Minister Hesham Kandil crossed into Gaza before midday, local time, through the only border post with Egypt, heavily guarded by security personnel wearing flak jackets and carrying assault rifles.

He was greeted by Gaza's Hamas prime minister, Ismail Haniyeh, who ventured out in public for the first time since Israel launched the offensive Wednesday.

In one chaotic moment, a man rushed toward the two leaders, shouting as he held up the body of a 4-year-old boy who Hamas said was killed in an Israeli airstrike — a claim Israel denied.

Fighting to hold back tears, Kandil told reporters that the Israeli operation must end.

"What I saw today in the hospital, the wounded and the martyrs, the boy ... whose blood is still on my hands and clothes, is something that we cannot keep silent about," he said.

Israel vociferously denied carrying out any form of attack in the area since the previous night. The pace of cross-border fighting quickly resumed after the Egyptian leader's departure.

Egypt 'will not leave Gaza on its own'

Three days of fierce fighting between Israel and Gaza militants has widened the instability gripping the region, straining already frayed Israel-Egypt relations.

Egypt's president Mohammed Morsi says Egypt "will not leave Gaza on its own" and warned the "aggressor to stop the bloodshed or face the wrath" of Egypt's new leadership and institutions.

Israeli soldiers prepare armoured personnel carriers at an area near the border with the Gaza Strip on Friday, in a sign that violence could escalate further with Palestinian militants. (Ronen Zvulun/Reuters)

Morsi spoke on Friday at a mosque near his house on the outskirts of Cairo. The sermon was his harshest condemnation yet of the Israeli offensive.

In fact, a senior Hamas official said that Egypt, which often mediates between Hamas and Israel, was working behind the scenes to arrange a truce.

The official, speaking on condition of anonymity, said Hamas was demanding an end to the offensive, limits on Israeli ground activities along the border, a permanent halt in assassinations of Hamas leaders and an end to Israel's blockade of Gaza.

Egypt said Kandil's three-hour visit Friday was meant as a show of solidarity with the Palestinian territory's militant Hamas rulers.