President Donald Trump waived US federal restrictions on foreign ships' transport of cargo to hurricane-ravaged Puerto Rico today.
He tweeted that relief supplies are getting through.
"The electric power grid in Puerto Rico is totally shot. Large numbers of generators are now on Island. Food and water on site," he wrote.
Hurricane Maria hit the US territory more than a week ago, leaving it without power and its roughly 3.4 million residents short on fuel and other supplies.
Mr Trump came under withering fire earlier this week for what some critics said was his slow response to an escalating humanitarian crisis.
Republicans and Democrats had urged Mr Trump to waive a little-known federal law called the Jones Act that prohibits foreign-flagged ships from shuttling goods between US ports.
Trump spokeswoman Sarah Huckabee Sanders said Mr Trump was responding to a request from Puerto Rico governor Ricardo Rossello, and that the waiver would go into effect immediately.
Mr Rossello responded on Twitter to Mr Trump's action: "Thank you @POTUS."
Advocates who pressed for the waiver have said it could get desperately needed supplies delivered to the island more quickly and at less cost.
Acting Homeland Security Secretary Elaine Duke had waived the law earlier this month to help ease fuel shortages in the Southeast following hurricanes Harvey and Irma.
That order included Puerto Rico, but expired last week shortly after Maria struck.
The Trump administration had said a waiver was not needed for Puerto Rico because there were enough US flagged ships available to ferry goods to the island.
Mr Trump told reporters yesterday that his administration was looking at a new waiver, but he said: "We have a lot of shippers and a lot of people and a lot of people that work in the shipping industry that don't want the Jones Act lifted."
Senator John McCain, in a tweet, noted the administration "has finally waived" the Jones Act.
"Now Congress must repeal this law to aid long-term recovery," he said.