(adds ICRC on humanitarian situation in new paras 6-12)

GENEVA, Sept 21 (Reuters) - U.N. High Commissioner for Human Rights Louise Arbour urged Israel on Friday to exercise restraint in its dealings with the Hamas-led Gaza Strip.

In a statement voicing concern at Israel's decision on Wednesday to declare Gaza an "enemy entity", Arbour said reducing fuel and power to the coastal territory would place an "unbearable burden" on its 1.5 million people.

The former United Nations war crimes prosecutor and Canadian Supreme Court judge also condemned the indiscriminate firing of rockets into Israel by Palestinian militants in Gaza.

Arbour reminded Israel of its obligations under international humanitarian and human rights law not to use disproportionate means or resort to collective punishment.

Gaza had already paid a "heavy price" from daily violence, isolation and deprivation, she said.

After three months of nearly complete closure, the humanitarian situation in the Gaza Strip is "critical", Angelo Gnaedinger, director-general of the International Committee of the Red Cross (ICRC), said on Friday on return from Gaza.

The agricultural sector is particularly affected, as most produce is grown for export, while most industrial and other businesses have had to close down, the ICRC said in a statement.

The operation of Gaza's water and sewage facilities has been impaired by military incursions, import restrictions, extensive damage to Gaza power station and interruptions to fuel supplies, the Swiss-based aid agency said.

An ICRC assessment of nine hospitals in the Gaza Strip found that the hospital infrastructure is deteriorating rapidly, it said.

"Many facilities, diagnostic machines and other equipment are either out of order or in bad condition, as it is no longer possible to maintain them properly," the ICRC said.

The transfer of Gaza patients allowed to go into Israel, Jerusalem and the West Bank for medical treatment had slowed to a "trickle", compared with the previous daily rate of 30 to 40 since the Hamas takeover in June, according to the ICRC.

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