The US military has begun construction on a pier meant to boost deliveries of desperately needed aid to Gaza, the Pentagon said on Thursday.

The small coastal territory has been devastated by more than six months of Israeli bombardment and ground operations against Hamas militants, leaving the civilian population in need of humanitarian assistance to survive.

"I can confirm that US military vessels... have begun to construct the initial stages of the temporary pier and causeway at sea," Pentagon spokesman Major General Pat Ryder told journalists.

Indications are that the pier will be operational in early May, and "everything is on course at this point," he said.

Highlighting the dangers in Gaza, Ryder said that the Pentagon was tracking "some type of mortar attack" causing minimal damage near the area onshore where the aid will eventually land.

"It's important also to highlight that this occurred before any US forces... started moving anything," he said.

The Israeli defence ministry body responsible for Palestinian civil affairs, COGAT, said that militants fired mortars the day before at an unspecified humanitarian work site in northern Gaza during a visit by UN personnel, though no casualties were reported.

Plans to build an offshore platform for the transfer of aid from larger to smaller vessels and a pier to bring it ashore were first announced by the United States in early March as Israel held up deliveries of assistance by ground.

US officials have said the effort will not involve "boots on the ground" in Gaza, but American troops will come close to the beleaguered territory as they construct the pier, for which Israeli forces are to provide onshore security.