US President Donald Trump is finally tweeting about hurricane-ravaged Puerto Rico - but his message has sparked outrage among residents struggling with the devastation.

Hurricane Maria slammed into the US territory of 3.4 million people last week, the most powerful hurricane to hit Puerto Rico in almost 90 years, destroying homes and knocking out power.

Amid mounting criticism that his administration wasn't doing enough to help, Mr Trump linked Puerto Rico's recovery from Hurricane Maria to its $US72 billion debt crisis.

Residents at La Perla community in Old San Juan comfort one another as the community recovers from Hurricane Maria, in San Juan, Puerto Rico. Photo / AP
Residents at La Perla community in Old San Juan comfort one another as the community recovers from Hurricane Maria, in San Juan, Puerto Rico. Photo / AP

"Texas & Florida are doing great but Puerto Rico, which was already suffering from broken infrastructure & massive debt, is in deep trouble," he tweeted on Monday EST.

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He said he will visit the Caribbean country on Tuesday.

Asked about the tweets on CNN's New Day, San Juan Mayor Carmen Yulín Cruz said: "Regarding President Trump, with all due respect, these are two different topics. One topic is the massive debt, which we know we have and it's been dealt with. But you don't put debt above people, you put people above debt. ...

"When somebody is in need, when somebody is in dire need, when somebody is in a life or death situation, there's a human, moral imperative to deal with that situation first and then deal with any other situations coming your way."

Cruz said Puerto Rico was in the grip of a "humanitarian crisis", with the death toll at 16.

"We are finding dialysis patients that have not been able to contact their providers. We are having to transport them in near-death conditions. We are finding people whose oxygen tanks are running out, because our small generators now don't have any diesel, and disabled people, they live alone and can't just walk somewhere," Cruz said.

A lone chicken walks amongst dead birds on a poultry farm in the aftermath of Hurricane Maria in Aibonito, Puerto Rico. Photo / AP
A lone chicken walks amongst dead birds on a poultry farm in the aftermath of Hurricane Maria in Aibonito, Puerto Rico. Photo / AP

"Every time we find a person that is gasping for air - I am not painting a poetic picture, I am telling you I have seen them and held them in my arms - and helped them into an ambulance, and every time we do that we get a little frail, and of course we get a little afraid.

"But we also get a lot more resolve to push on and move on and to do whatever - our bodies are so tired, but our souls are so full of strength that we will get to everyone we can get to. But the situation in the hospitals is something that has to be dealt with completely and on a sustained basis."

Puerto Rico Governor Ricardo Rossello has called for federal aid to flow quickly to the island.

"Given Puerto Rico's fragile economic recovery prior to the storms, we ask the Trump Administration and US congress to take swift action to help Puerto Rico rebuild," Rossello said in a statement on Monday.

National Guardsmen arrive at Barrio Obrero in Santurce to distribute water and food among those affected by the passage of Hurricane Maria, in San Juan, Puerto Rico. Photo / AP
National Guardsmen arrive at Barrio Obrero in Santurce to distribute water and food among those affected by the passage of Hurricane Maria, in San Juan, Puerto Rico. Photo / AP

Critics have said the Trump administration is not acting fast enough, with some saying that although Puerto Rico is not a state, its people are US citizens and should be treated fairly.

The island is a US commonwealth and its inhabitants do not have a vote in presidential elections or full representation in congress.

Among those critics is singer Marc Anthony, whose family is from Puerto Rico.

In a strongly-worded Twitter post on Monday, he said Mr Trump should stop dwelling on a controversy involving National Football League players and the national anthem and, "Do something about our people in need in #PuertoRico. We are American citizens too."

White House spokeswoman Sarah Sanders denied the administration had been slow to respond.