The US and allies are scrambling to pull together a complex system that will move tonnes of humanitarian aid into Gaza by sea.

Nearly two months after President Joe Biden gave the order, US Army and Navy troops are assembling a large floating platform several miles off the Gaza coast that will be the launching pad for deliveries.

"As of today, we are over 50 per cent complete on setting up the pier," deputy Pentagon press secretary Sabrina Singh told journalists at a press conference on Wednesday, local time.

But any eventual aid distribution — which could start as soon as early May — will rely on a complicated logistical and security plan with many moving parts and details that are not yet finalised.

The relief is desperately needed, with the UN saying people in Gaza are on the brink of famine. But there are still widespread security concerns. Some aid groups have said that with so much more needed, the focus should instead be on pushing Israel to ease obstacles to the delivery of aid on land routes.

Setting up the system is expected to cost at least $US320 million, ($490 million) the Pentagon said on Monday.

Here's how it will work:

It all starts in Cyprus

Humanitarian aid bound for Gaza through the maritime route will be delivered by air or sea to Cyprus, an island at the eastern edge of the Mediterranean Sea.

Cyprus Foreign Minister Constantinos Kombos said the aid will undergo security checks at Larnaca port.

Using that one departure point will address Israel's security concerns that all cargo be inspected to ensure that nothing is loaded on ships that Hamas could use against Israeli troops.

The screening will be strict and comprehensive, including the use of mobile X-ray machines, according to a Cyprus government official who spoke on condition of anonymity to publicly disclose details about the security operation.

The process will involve Cypriot customs, Israeli teams, the US and the United Nations Office for Project Services.

An American military official said the US has set up a coordination cell in Cyprus to work with the government there, the US Agency for International Development and other agencies and partners.

The military official said the group will focus on coordinating the collection and inspection of the aid.

The official also spoke on condition of anonymity to discuss operation details.

The floating pier

Once the pallets of aid are inspected, they will be loaded onto ships — mainly commercial vessels — and taken about 321 kilometres to the large floating pier being built by the US military off the Gaza coast.

There, the pallets will be transferred onto trucks that in turn will be loaded onto two types of smaller Army boats — Logistic Support Vessels, or LSVs, and Landing Craft Utility boats, LCUs.

The US military official said the LSVs can hold 15 trucks each and the LCUs can hold about five.

The Army boats will then shuttle the trucks from the pier to a floating causeway, which will be several miles away and anchored into the beach by Israeli Defense Forces (IDF)

Since Mr Biden has made clear that no US forces will step foot in Gaza, the troops doing the construction and driving and crewing the boats will be housed and fed on other ships offshore near the large floating pier.

The British Royal Navy support ship RFA Cardigan Bay will provide accommodation for hundreds of US sailors and soldiers working to establish the pier.

Another contracted ship will also be used for housing, but officials did not identify it.

Causeway to be installed

The small army boats will sail to the two-lane, 550-metre causeway.

The US military official said an American Army engineering unit has teamed up with an Israeli engineering unit in recent weeks to practice the installation of the causeway, training on an Israeli beach just up the coast.

The UK Hydrographic Office also has worked with the US and the Israeli military to analyse the shoreline and prepare for the final installation.

US vessels will push the floating causeway into place, shoving it into the shoreline, where the IDF will be ready to secure it.

Trucks loaded with the pallets of aid will drive off the Army boats onto the causeway and down to a secure area on land where they will drop off the aid and immediately turn around and return to the boats.

The trucks will repeat that loop over and over, and they will be confined to that limited route to maintain security.

They will be driven by personnel from another country, but US officials have declined to say which one.

Distribution to aid agencies and civilians

Aid groups will collect the supplies for distribution on shore, at a port facility built by the Israelis just south-west of Gaza City.

Officials say they expect about 90 truckloads of aid a day initially and that it will quickly grow to about 150 a day.

The UN is working with USAID to set up the logistics hub on the beach.

There will be three zones at the port:

• One controlled by the Israelis where aid from the pier will be dropped off.
• One where the aid will be transferred.
• A third where Palestinian drivers contracted by the UN will wait to pick up the aid before taking it to distribution points.

Aid agencies, however, say this maritime corridor isn't enough to meet the needs in Gaza and must be just one part of a broader Israeli effort to improve sustainable, land-based deliveries of aid to avert famine.

The groups, the UN the US and other governments have pointed to Israel's aid restrictions and its failure to safeguard humanitarian workers as reasons for the reduction in food shipments through land crossings, although they credit Israel with making some improvements recently.

US Gaza envoy David Satterfield said last week that only about 200 trucks a day were getting into Gaza, far short of the 500 that international aid organisations say are needed.

Security onshore and offshore

A key concern is security — both from militants and the Israeli military, which has been criticised for its killing of aid workers.

Sonali Korde, a USAID official, said key agreements for security and handling the aid deliveries are still being negotiated.

Those include how Israeli forces will operate in Gaza to ensure that aid workers are not harmed.