William Hague says Hamas bears responsibility for escalating violence in the region, as Israel's prime minister says the army is prepared for a "significant widening" of its operation.

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William Hague said he was "gravely concerned" by the situation in Gaza and southern Israel.

Thirteen Palestinians, including children and Hamas's military chief Ahmed al-Jaabari, were reportedly killed in an Israeli attack on Gaza. Hamas retaliated by firing rockets at Israel and three people were killed, according to the Israeli army.

"Hamas bears principal responsibility for the current crisis," said Mr Hague. "Hamas and other armed groups in Gaza should cease attacks against Israel immediately."

He added: "I also strongly urge Israel to do their utmost to reduce tension, avoid civilian casualties and increase the prospects for both sides to live in peace."

The UN Security Council held an emergency meeting on Wednesday night to discuss Israel's attacks on Gaza, but it took no action.

Israeli Prime Minister Benjamin Netanyahu said on Thursday that he is considering a ground assault and that the army is prepared for a "significant widening" of its operation in the Gaza Strip. Benjamin Netanyahu told reporters that Israel had "made it clear" it won't tolerate continued rocket fire on its civilians and that action will be taken if Hamas did not receive the message.

Egypt defends Palestine

The attack was Israel's most intense on Gaza since a full-scale invasion four years ago. However much has changed in the region since then: following a popular uprising last year, Egypt is now governed by the Muslim Brotherhood, which has close ties with Hamas and shares a border with Gaza. Egypt's new Islamist government has so far honored a 1979 peace deal with Israel. But it is far cooler to Israel than ousted President Hosni Mubarak was.

Egypt's President Mohammed Morsi condemned the Israeli offensive and Cairo has recalled its ambassador from Israel. He said in a statement that he stands by the Palestinian people and will hold an emergency cabinet meeting to discuss Israel's "aggression" against Gaza.

The new Islamist government brokered a truce between the two sides on Tuesday only to see it shattered a day later when Israel assassinated the top Hamas military commander.

In an initial protest, Egypt recalled its ambassador to Israel on Wednesday. Egypt's Foreign Minister Mohammed Kamel Amr also spoke with US Secretary of State Hillary Clinton late, asking for "immediate US intervention to stop the Israeli aggression," said the foreign ministry.

Earlier this month, Palestinian authorities said they planned to approach the United Nations at the end of November to request a diplomatic upgrade for Palestine to "observer" status. If successful, this would mean Palestine could attempt to prosecute Israeli ministers and other for war crimes, said author and blogger Antony Lowenstein

The attack on Gaza also comes ahead of elections in Israel in January.

'Open the gates of hell'

The United States condemned Hamas, shunned by the west as an obstacle to peace, for its refusal to renounce violence and recognise Israel.

"There is no justification for the violence that Hamas and other terrorist organisations are employing against the people of Israel," said Mark Toner, deputy state department spokesman.

Hamas has said the killing of its top commander, Ahmed Al-Jaabari, would "open the gates of hell" for Israel. It also appealed to neighbouring Egypt to halt the "barbaric" assault.

Iran issued a statement on Thursday morning saying that Israel is guilty of "organised terrorism".

"Iran considers the criminal act of Israeli military forces in killing civilians as organised terrorism and strongly condemns it," foreign ministry spokesman Ramin Mehmanparast said, according to the semi-official Fars news agency.

In Lebanon, the Iranian-backed Shi'ite militia Hezbollah denounced Israel's strikes on Gaza as "criminal aggression" and called on Arab states to "stop the genocide".