Hurricane Maria hit the US island territory before dawn Wednesday as a Category Four storm, leaving vast tracts of the island with power or telephone coverage.
The five living former US presidents extended their "One America Appeal" -- set up in the wake of Hurricanes Harvey in Texas and Irma in Florida -- to help with the devastation in Puerto Rico.
The White House denied it had been slower to act in Puerto Rico than in storm-battered areas on the US mainland, although Trump himself spent part of the day sending multiple tweets fueling a feud with sportsmen refusing to stand for the national anthem.
"We've done unprecedented movement in terms of federal funding to provide for the people of Puerto Rico and others that have been impacted (by) these storms," said spokeswoman Sarah Huckabee Sanders.
"We'll continue to do so and continue to do everything that we possibly can under the federal government to provide assistance."
Engineers say it could take months for power to be fully restored in Puerto Rico.
After Hurricane Harvey hit Texas, Trump traveled to the state twice in one week.
He also traveled to Florida after Irma, handing out sandwiches and stressing that restoring electricity was a priority.
Trump spoke to Puerto Rico's Governor Ricardo Rossello and the governor of the US Virgin Islands by phone over the weekend, and vowed to visit both places -- although no date has yet been set.