Donald Trump has warned hurricane-devastated Puerto Rico that the United States Government cannot keep sending help "forever" and suggested the US territory was to blame for its financial struggles.

His broadsides triggered an outcry from Democrats in Washington and officials on the island, which has been reeling since Hurricane Maria struck three weeks ago, leaving death and destruction in an unparalleled humanitarian crisis.

In a series of tweets, the President said "electric and all infrastructure was disaster before hurricanes".

He blamed Puerto Rico for its looming financial crisis and "a total lack of accountability".

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The tweets conflicted with Mr Trump's past statements on Puerto Rico, including those made during an event last week honouring the heritage of Hispanics, in which the President said: "We will be there all the time to help Puerto Rico recover, restore, rebuild."

San Juan Mayor Carmen Yulin Cruz, with whom Mr Trump has had a running war of words, tweeted that the President's comments were "unbecoming" to a commander-in-chief and "seem more to come from a 'hater in chief'".

"Mr President, you seem to want to disregard the moral imperative that your administration has been unable to fulfil," the Mayor said.

"Tweet away your hate to mask your administration's mishandling of this humanitarian crisis."

Democrats said Mr Trump's tweets were deplorable, given that the 3-million-plus US citizens on Puerto Rico were confronting the kind of hardships that would draw howls of outrage if they affected a state.

One-third of the island lacks clean running water and just 8 per cent of its roads are passable, according to government statistics.

"It is shameful that President Trump is threatening to abandon these Americans when they most need the Federal Government's help," the second-ranking House Democrat Steny Hoyer said.

Responders trying to 'work themselves out of a job'

Sorry, this video has expired Donald Trump says Puerto Rico has "thrown our budget a little out of whack"

The debate played out as the House passed, on a sweeping 353-69 vote, a $36.5 billion ($46.6 billion) disaster aid package that includes assistance for Puerto Rico's financially-strapped Government.

House Speaker Paul Ryan said the Government needed to ensure that Puerto Rico can "begin to stand on its own two feet" and said the US has "got to do more to help Puerto Rico rebuild its own economy".

White House chief of staff John Kelly said the military and other emergency responders were trying very hard to "work themselves out of a job".

Reassuring the island, General Kelly said the US would "stand with those American citizens in Puerto Rico until the job is done".

Forty-five deaths in Puerto Rico have been blamed on Maria, about 85 per cent of Puerto Rico residents still lack electricity and the Government said it hoped to have electricity restored completely by March.

Both Mr Trump and Vice President Mike Pence visited the island last week to reaffirm the US commitment to the island's recovery.

The island's recovery will likely cost more than $US30 billion. ( AP: Ramon Espinosa )

During his visit Mr Trump highlighted the island's relatively low death toll — which at that stage stood at 16.

The financial cost of the disaster was not far from his mind though, with the President praising the federal assistance so far in Puerto Rico, but saying the disasters were straining the boundaries of the US budget.

"I hate to tell you, Puerto Rico, but you've thrown our budget a little out of whack because we've spent a lot of money on Puerto Rico," he said.

"And that's fine, we've saved a lot of lives."

After years of economic challenges, Puerto Rico was already in the process of restructuring much of its $74 billion in debt before the hurricane struck.

The financial situation is more complicated than Mr Trump's tweets suggest.

Puerto Rico lost population and jobs after Congress eliminated special tax breaks in 2006, making it more difficult to repay its debts.

Yet lenders continued to extend credit to Puerto Rico despite its economic struggles, while pension costs strained Puerto Rico's government and its infrastructure deteriorated.

AP