US President Donald Trump has pledged to spare no effort to help Puerto Ricans recover from Hurricane Maria's ruinous aftermath even as San Juan's mayor, her voice breaking with rage, accused his administration of, "killing us with the inefficiency".

Key points: Mayor tells Trump "we are dying, and you are killing us with the inefficiency"

Mayor tells Trump "we are dying, and you are killing us with the inefficiency" Thousands get water, rationed food as aid bottleneck eases

Thousands get water, rationed food as aid bottleneck eases Trump draws attention to Puerto Rico's debt while discussing aid

Mayor Carmen Yulin Cruz implored Mr Trump from afar to, "make sure somebody is in charge that is up to the task of saving lives", while the President asserted US officials and emergency personnel were working all-out against daunting odds, with "incredible" results.

Mr Trump's acting homeland security secretary, Elaine Duke, visited the island on Friday (local time), surveying the ravaged landscape by helicopter in an hour-long tour, passing still-flooded streets, twisted billboards and roofs with gaping holes, and offering encouragement to some of the 10,000 emergency personnel she says the US government has on the ground.

Puerto Rico is suffering badly after being whacked by Hurricane Maria. ( Twitter: Manuel E. Rivera )

Ms Duke tried, too, to move on from the remarks she made a day earlier in which she called the federal relief effort a "good-news story". But on that front, she ran into winds as fierce as Maria.

"We are dying, and you are killing us with the inefficiency," Ms Cruz said in a news conference. "I am begging, begging anyone that can hear us, to save us from dying."

Sorry, this video has expired Puerto Rico residents line for fuel, water ( North America correspondent Zoe Daniel )

Thousands more Puerto Ricans got water and rationed food on Friday as an aid bottleneck began to ease.

Telecommunications are back for about 30 per cent of the island, nearly half of the supermarkets have reopened, at least for reduced hours, and about 60 per cent of the petrol stations are pumping.

But many remain desperate for necessities, most urgently water, long after the September 20 hurricane.

More aid is starting to reach the streets of Puerto Rico. ( AP: Carlos Giusti )

Mr Trump said Puerto Rico was "totally unable" to handle the catastrophe on its own.

"They are working so hard, but there's nothing left," he said. "It's been wiped out."

He said the Government was, "fully engaged in the disaster and the response and recovery effort".

He said he was not aware of Ms Duke's "good-news" remark.

"I haven't heard what she said," he told reporters. "I can tell you this: We have done an incredible job considering there's absolutely nothing to work with."

Trump draws attention to debt

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Yet even in voicing solidarity and sympathy with Puerto Rico, he drew attention again to the island's pre-hurricane debt burden and infrastructure woes, leaving doubt how far Washington would go to make the US territory whole.

"Ultimately the Government of Puerto Rico will have to work with us to determine how this massive rebuilding effort — it will end up being one of the biggest ever — will be funded and organised, and what we will do with the tremendous amount of existing debt already on the island," he said.

"We will not rest, however, until the people of Puerto Rico are safe."

AP