UN Secretary General Ban Ki Moon calls on Israel to exercise "maximum restraint" to avoid further bloodshed in Israel and the Palestinian territories.

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The statement, made by Mr Ban's spokesman, came as Israel announced it is closing three roads around the Gaza strip.

"The secretary-general is extremely concerned about the continued violence in Gaza and Israel, and deeply worried by the rising cost in terms of civilian lives," UN spokesman Martin Nesirky said.

"Rocket attacks are unacceptable and must stop at once. Israel must exercise maximum restraint."

Earlier in the day, Egypt's prime minister arrived in Gaza in an apparent display of solidarity with Hamas. Hisham Qandil first met Hamas' Deputy Prime Minister Ziad al-Zaza and later Hamas Prime Minister Ismail Haniyeh. The Arab League is to meet on Saturday to discuss the conflict.

A hoped-for ceasefire for the visit failed to hold with both sides accusing the other of breaking it. Rockets fired from Gaza hit several sites in southern Israel and the Israeli air force attacked the house of Hamas's commander for southern Gaza. Medics said it killed two people, one of them a child, raising the Palestinian death toll since Wednesday to 22, which includes seven children.

On the Israeli side, a Hamas rocket killed three Israelis in the town of Kiryat Malachi on Thursday morning.

Read more about the reasons behind the increase in violence in Gaza

The armed wing of Hamas said it fired a Qassam rocket at Jerusalem, according to Reuters. Accprding to Israeli radio the rocket landed in the Jerusalem area but no casualties were reported.

After Mr Qandil's visit, a rocket from Gaza landed in the sea near Tel Aviv, around 30 miles from Gaza, demonstrating that Hamas has long-range capability.

Around 450 Gaza sites have been hit by Israel since Wednesday, and more than 200 rockets fired from Gaza have hit Israeli communities.

Foreign Secretary William Hague on Thursday said that Hamas "bears principal responsibility for the current crisis".

Tel Aviv under fire

The escalation in violence was triggered on Wednesday when Israel killed Hamas' military chief, Ahmed al-Jaabari, in an air strike on his car. Israel then began shelling the coastal enclave from land, air and sea.

The offensive on Gaza further intensified later on Thursday when Hamas and Islamic Jihad fired two rockets at Tel Aviv in the first such attack on Israel's commercial capital since 1991.

The Israeli armed forces are preparing to mobilise up to 75,000 reserve troops, in a sign that a ground offensive in Gaza is being considered.

Egypt's new Islamist president, Mohamed Mursi, viewed by Hamas as a protector, led a chorus of denunciation of the Israeli strikes by allies of the Palestinians. However, recent violence stemming from Egypt's Sinai peninsula which has been blamed on Islamist groups, has strained relations between Egypt and Hamas.

Mr Mursi faces domestic pressure to act tough and the country shares a border with Gaza. But Egypt gets $1.3bn a year in US military aid and looks to Washington for help with its ailing economy.

This ties Mursi somewhat, despite his need to show Egyptians that his policies differ from those of his US-backed predecessor, Hosni Mubarak.