Hurricanes Maria and Irma killed 13 people on the island - with Maria almost completely destroying telecommunication networks last week.
"Texas & Florida are doing great but Puerto Rico, which was already suffering from broken infrastructure & massive debt, is in deep trouble," Trump tweeted late on Monday.
"It's old electrical grid, which was in terrible shape, was devastated. Much of the Island was destroyed, with billions of dollars owed to Wall Street and the banks which, sadly, must be dealt with. Food, water and medical are top priorities - and doing well. #FEMA."
The White House earlier denied it had been slower to act following Hurricane Maria in overwhelmingly Hispanic Puerto Rico than in the aftermath of Hurricanes Irma and Harvey on the US mainland.
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But Representative Adam Smith, the top Democrat on the House Armed Services Committee, condemned the Trump administration's response to the crisis as "wholly inadequate."
"A territory of 3.5 million American citizens is almost completely without power, water, food and telephone service, and we have a handful of helicopters involved in DOD's response. It's a disgrace," he said.
Many Puerto Ricans have already started their own cleanup operations amid apocalyptic scenes of destruction, with some small shops and restaurants reopening with the help of generators.
But long lines remain at supermarkets and gas stations -- with water, gas and ice all rationed.
Trump on Friday called for NFL players demonstrating against racial inequality during the national anthem to be fired, triggering a row in which leading players across the country knelt in protest over the weekend.
