By Michelle Nichols

UNITED NATIONS (Reuters) -United Nations chief Antonio Guterres on Friday said Israel has only authorized for Gaza what "amounts to a teaspoon of aid when a flood of assistance is required" and again signaled that the U.N. won't take part in a new U.S.-backed distribution plan.

"Without rapid, reliable, safe and sustained aid access, more people will die – and the long-term consequences on the entire population will be profound," Guterres told reporters.

Israel says about 300 trucks of aid have entered Gaza via the Kerem Shalom crossing since it lifted an 11-week blockade on Gaza on Monday, but Guterres said that so far only about a third of those truckloads have been transported from the crossing to warehouses within Gaza due to insecurity.

Israel has allowed aid deliveries by the U.N. and other aid groups to briefly resume until a new U.S.-backed distribution model - run by the newly created Gaza Humanitarian Foundation - is up and running by the end of the month. The U.N. says the plan is not impartial or neutral, and that it will not be involved.

Israel said its blockade had been aimed in part at stopping Palestinian militants Hamas from diverting and seizing aid supplies. Hamas has denied stealing aid. The GHF plan involves using private security contractors to transport aid to so-called secure hubs for distribution by civilian humanitarian teams.

"The United Nations has been clear: We will not take part in any scheme that fails to respect international law and the humanitarian principles of humanity, impartiality, independence and neutrality," Guterres said.

The U.N. and its partners have a plan to get the aid needed into Gaza, he said.

"The supplies – 160,000 pallets, enough to fill nearly 9,000 trucks – are waiting," Guterres said. "This is my appeal for life-saving aid for the long-suffering people of Gaza: Let's do it right. And let's do it right away.

(Reporting by Michelle Nichols; Editing by Caitlin Webber and Philippa Fletcher)