By Peter Apps

LONDON, June 25 (Reuters) - Unloaded from trucks and ferried across the no man's land between Israel and Gaza, aid is trickling in to the Hamas-controlled strip but key checkpoints remain shut and relief workers warn of a rising crisis.

Even before Hamas Islamists took control in violent fighting with their secular Fatah counterparts earlier in the month, triggering the closure of front-line crossing points, aid agencies were warning of growing hardship for ordinary people.

All sides, including Israel, say they are committed to getting essential aid to the 1.5 million Gaza residents. However, the cargo checkpoint at Karni remains shut for security reasons and the United Nations World Food Programme (WFP) says the remaining supply links are tenuous.

"Put simply, the noose is tightening at the moment," WFP Gaza emergency coordinator Kirsty Campbell told Reuters. "There is not enough getting in. We hope by the end of the week there may be. But it is quite an ambitious task."

Prices of essential foods had risen by between a half and a third, she said, with authorities in Gaza taking action to control the supply of flour and prevent traders from hoarding. Farmers are short of supplies and hospitals of medicines.

Aid workers say an international boycott of the Hamas-led government prevented public sector workers from receiving their full salaries, deepening poverty in the strip.

Donors have now relaxed the boycott on President Mahmoud Abbas's administration on the West Bank, but the United States and Israel say the embargo will remain in place on the Hamas administration in Gaza.

That outrages some aid groups.

Oxfam says the boycott has left essential water equipment waiting at the border for months and that, as a result, a key sewage works could overflow, swamping up to 10,000 people and contaminating water for 300,000.

AVOIDABLE CRISIS

"The international community is closing its eyes to its humanitarian obligations and allowing the suffering to intensify," said the director of Oxfam International, Jeremy Hobbs. "Aid is being drip fed across the border. We urge the key players to resolve what has been a completely avoidable crisis."

Some aid groups say the boycott may risk further radicalising Gaza residents, but analysts say there is little chance of international donors aiding a de facto Hamas administration still seen as threatening Israel.

Abbas and his Fatah administration are likewise keen to isolate Hamas further by withholding all but essential aid, some Western diplomats said.

The WFP said it had managed to get about 650 tonnes of food into Gaza last week and hoped to push 11 truckloads through the small Sufa crossing point on Monday as Palestinian and Israeli leaders met in Egypt.

It is a tortuous process. A gate on the Israeli side is opened to allow pallets of food to be unloaded into no man's land by fork lift truck in the morning, then the gate on the Gaza side opens to allow the food to be put on trucks in the afternoon.

Another small crossing point had also been due to be open on Monday but was closed after a security incident, the WFP's Campbell said.

Before the Hamas takeover, aid and commercial supplies -- essential to maintaining ordinary trade and economic activity in Gaza -- would have passed through the main Karni checkpoint in containers. But agreeing procedures between Hamas and Israel to allow it to reopen seems difficult or impossible.

"All it would take is for one rocket to fall on Sufa and it would be out of action," Campbell said. "We need to make sure we have as many corridors as possible. Every day they are out of action at the moment causes serious difficulties."

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