Gaza has seen one of its heaviest nights of bombardment in three weeks, after Israel's prime minister warned of a long conflict ahead.Gaza's only power plant was damaged as Israel carried out 60 air
strikes, also targeting sites associated with Hamas, the group which controls Gaza.
At least 60 people died in Gaza, possibly including UN staff members.
An Israeli military spokesman said the strikes signalled a "gradual increase in the pressure" on Hamas.
In a televised address on Monday, Prime Minister Benjamin Netanyahu stressed the need to destroy tunnels dug under the Gaza-Israel border, to prevent militants infiltrating Israel.
UN Secretary General Ban Ki-moon expressed concern that Israel was reported to be dropping leaflets warning residents in northern Gaza to leave.
Palestinian officials say 1,115 Palestinians, most of them civilians, have been killed in the fighting since 8 July while Israel has lost 53 soldiers and three civilians - two Israelis and a Thai worker.
UN Relief and Works Agency spokesman Chris Gunness said in a tweet that a number of staff members had reportedly been killed. The UN is currently caring for 182,604 Palestinians in its 82 shelters in Gaza, he said.
In another development, Iran's Supreme Leader, Ayatollah Ali Khamenei, accused Israel of acting like a "rabid dog" and called on Muslims to arm Palestinians to enable them to fight back against "genocide".