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.

He said UN agencies there did not have the resources to help an extra influx of people. Israel says it issues such warnings to try to avoid civilian casualties.

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".

Israeli forces struck by air, sea and land, lighting up the night sky with flare bursts and leaving long plumes of smoke trailing over Gaza City in the morning.

Shells hit a fuel tank supplying the strip's only power station and forced it to shut down, putting already restricted supplies to the city and other areas in jeopardy.

Fifty-five houses were destroyed in the bombing, with people buried under rubble in at least three of them, Palestinian security sources told the BBC.

The unoccupied house of former Hamas Prime Minister Ismail Haniyeh was destroyed.

"The destruction of stones will not break our will and we will continue our resistance until we gain freedom." he was quoted as saying on a Hamas website afterwards.

Nine other buildings were targeted, and three mosques and four factories were also destroyed.

As well as Hamas TV and radio stations, government buildings, including the finance ministry and a compound belonging to the interior ministry, were attacked.

Gaza's port was also destroyed, Palestinian security sources told the BBC, and two schools and a kindergarten were on fire after being hit.

According to the health ministry, in the past 24 hours 110 people were killed, 60 of them since midnight local time.

Seven families were "wiped out" during the night, the ministry said.

The military wing of Hamas said it had fired 14 rockets.

Five Israeli soldiers were killed on Monday when militants infiltrated the border, while a mortar bomb killed four earlier and a tenth died in a clash in southern Gaza, the Israel Defense Forces said.

Lt-Col Peter Lerner, an Israeli military spokesman, told AP news agency on Tuesday that pressure was being increased on Hamas.

"Israel is determined to strike this organisation and relieve us of this threat," he said.

In his address on Monday night, Mr Netanyahu said Gaza had to be demilitarised in order to protect Israel.

"We will not finish the operation without neutralising the tunnels, which have the sole purpose of destroying our citizens, killing our children," he said.

Israel's Operation Protective Edge began on 8 July after a surge in militant rocket attacks.

A rally in support of the operation is planned for this evening in Tel Aviv.