(Adds Ban Ki-moon comment in paragraphs 6, 10-11)

By Louis Charbonneau

UNITED NATIONS, Jan 18 (Reuters) - Israel has a right to respond to security threats but should not collectively punish the Gaza population for rocket attacks from the Palestinian territory, the U.N. humanitarian affairs chief said on Friday.

The United Nations also criticized Israel's decision to close all border crossings with Gaza, preventing delivery of a U.N. aid shipment to the territory's 1.5 million people, most of whom depend on foreign aid.

"We all understand the security problems and the need to respond to that but collective punishment of the people of Gaza is not, we believe, the appropriate way to do that," said John Holmes, undersecretary-general for Humanitarian Affairs.

The deputy head of Israel's mission to the United Nations, Daniel Carmon, told Reuters Israel's actions were "what any responsible government would do when it is confronted as we are with this surge of violence and terrorism."

He gave no indication of when the closure would end.

U.N. Secretary-General Ban Ki-moon called "for an immediate cessation of Palestinian sniper and rocket attacks into Israel, and for maximum restraint on the part of the Israel Defence Forces," spokeswoman Michel Montas said in a statement.

Israel has killed at least 33 Palestinians in Gaza this week as part of what it describes as a stepped-up campaign to force Hamas to rein in militants who have fired more than 110 rockets into the Jewish state in the last three days alone.

It bombed the Hamas-run Interior Ministry in Gaza on Friday, killing one woman and injuring at least 30 others who were nearby, medical officials said.

Holmes said he was worried about the sealing of the border crossings because "they are the lifeline for the delivery of humanitarian assistance and other goods to Gaza."

U.N. PRESSURE ON ISRAEL AND HAMAS

Ban joined Holmes in urging Israel to end the closure, saying it cut off the population from fuel supplies needed to pump water and generate electricity for homes and hospitals.

"The closure will also cause further shortages of food, medical and relief items in the Gaza Strip," he said.

Holmes said the Israeli response was unwarranted.

"This kind of action against the people in Gaza cannot be justified, even by those rocket attacks," he said.

Carmon said Israel was "very much aware of the humanitarian situation in Gaza." The Israeli government has said that humanitarian goods would be allowed into the territory.

Holmes also urged the Palestinian militant group Hamas, which seized control of Gaza last June, to use its authority to put an end to the attacks against Israel.

"I'm calling on the Hamas leaders ... to do whatever they can to stop these attacks because they claim to be in control of Gaza there," Holmes said. "Therefore they have a responsibility to stop the attacks."

Israel has imposed strict curbs on non-humanitarian supplies to Gaza since Hamas's takeover.

But many essentials have been getting in, either with Israeli approval or through smuggling, though supplies are limited and prices have risen steeply.

Holmes said he worried the violence in Gaza could spin out of control, making a dire humanitarian situation even worse.

"I believe it is a (humanitarian) crisis already," he said. (Editing by Bill Trott)

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