By Stephanie Nebehay

GENEVA, June 22 (Reuters) - Aid agencies on Friday warned of a "major humanitarian crisis" in the Hamas-held Gaza Strip, where food stocks could run out in two weeks, unless Israel's cargo blockade eases further.

Goods are currently only able to trickle through Kerem Shalom crossing, whose capacity is 15 trucks a day, and it was vital to reopen Karni crossing, which can handle up to 200 trucks a day, they said.

Israel, the transit point for imports to the Gaza Strip, ordered its customs agents to hold back shipments after Hamas gunmen seized control of the enclave last week. On Thursday, it said medical equipment, food and other vital goods would be released if its importers proved they were relief items.

"The needs are growing ... Therefore food and other humanitarian supplies have to continue to enter Gaza if a major humanitarian crisis is to be averted," Simon Pluess of the U.N.'s World Food Programme (WFP) told a news briefing.

The WFP warned on Wednesday that the Gaza Strip could start running out of flour, rice, food oil and other commodities in 2-4 weeks unless Israel opened the border crossings. The WFP has managed to get 400 tonnes of food into Gaza so far this week, Pluess said.

But it was "quite unlikely" that the Karni crossing would reopen soon since it had suffered major structural damage during recent looting by Palestinians, he added.

The Kerem Shalom route from the south was becoming a major lifeline, Pluess said.

Israel wants to isolate Gaza's Hamas Islamists economically, diplomatically and militarily, while allowing funds and trade to flow to the Western-backed emergency government set up in the West Bank by Palestinian President Mahmoud Abbas.

The U.N. agency serving some 1 million Palestinian refugees in Gaza, or 72 percent of the population, said that Karni must be reopened urgently.

"The reopening of Karni is crucial to prevent food shortages in two weeks," said Matthias Burchard, Geneva representative of the U.N. Relief and Works Agency (UNRWA).

Veronique Taveau, of the U.N. Children's Fund (UNICEF), said it was delivering vaccines and emergency medical supplies to help prevent outbreaks of water-borne diseases in Gaza.

"The blockade of crossing points has made bringing in aid almost impossible. Medical stocks are reaching the critical level," she said.

Some two-thirds of families in Gaza already live below the poverty line, according to UNICEF which is also supplying fuel to run water and sanitation systems for 300,000 people.

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