HOMESTEAD, Fla. -- U.S. Sen. Bill Nelson accused the Trump administration of a "coverup" after officials denied him entry Tuesday to a detention centre for migrant children in South Florida where he had hoped to survey living conditions.

Nelson and U.S. Rep. Debbie Wasserman Schultz, both Florida Democrats, went to the contractor-run Homestead Temporary Shelter for Unaccompanied Children following reports it was receiving detained children who had arrived in the country illegally.

Wasserman Schultz said the facility was being used for an estimated 1,000 children, aged 13 to 17 -- most of whom arrived as unaccompanied minors and about 10 per cent of whom are children separated from their families at the border. She said two other South Florida facilities were being used for younger children.

"It is an affront as the senior senator of this state that an agency head would tell me that I do not have entrance into a federally funded facility where the lives and health of children are at stake," Nelson said.

President Donald Trump's immigration policies have drawn intense scrutiny following reports of the forced separation of migrant children from their parents. Democrats and some Republicans are urging an end to the practice at the U.S.-Mexico border. Thousands of children split from their families at that border are being held in government-run facilities.

Wasserman Schultz said her staff had spoken Tuesday with the Florida-based company, Comprehensive Health Services, contracted to run the facility. She said her staff was told the lawmakers would be "welcomed warmly and allowed into the facility."

But Nelson said Deputy Secretary of Health and Human Services Eric Hargan told him it would take two weeks for them to gain access.

"I think what they're doing is a coverup for the president," Nelson said.

Trump doesn't like the negative response he's received, even from fellow Republicans, Nelson said.

"Are they abusing these kids? Are they sleeping on the floor? Are they in cages, like we've seen in some videos?" Wasserman Schultz asked after being barred from the building.

The Florida facility is overseen by the U.S. Department of Health and Human Services. Department spokesman Kenneth Wolfe said in an email Monday that it had reopened as "a temporary unaccompanied alien children program facility." He did not provide further details.

Gov. Rick Scott's office, however, released documents Tuesday that showed that federal authorities in February notified state officials and members of Congress that the Homestead facility would be reopened. Federal authorities didn't give an exact date, but said the Homestead location would reopen after damage from Hurricane Irma was repaired. The release from HHS also stated that the facility would only be used for "unaccompanied alien children" detained by immigration officials.

Later Tuesday, Scott called on Trump's administration to stop separating the families. The Republican governor sent his request in a letter to Health and Human Services Secretary Alex Azar. Scott's letter also requested that federal authorities notify state officials when they bring into Florida migrant children who have been separated from their families.

Scott also wants to know what services are being provided to the children and whether they have had any health screenings. He said the information is needed to make sure that the children are being protected.

An Associated Press reporter was denied access to Azar while he visited a Miami hospital Tuesday to talk with patients about drug costs. Spokesman Gavin Smith barred the AP reporter from asking Azar about the immigration facility because an interview with the secretary had not been pre-arranged.

Several dozen children could be seen Tuesday playing soccer outside the building behind a chain link fence, mostly talking and shouting to each other in Spanish. Some did cartwheel flips while running across the field.

Security officials would not let reporters near the facility or provide details on conditions inside.

Mark Greenberg, a former head of the HHS Administration for Children and Families, said agency policy says requests to visit facilities for migrant children be submitted two weeks in advance. However, Greenberg said in the current state of heightened concern it behooves HHS to act rapidly on requests from lawmakers.

Greenberg said much of the reason for lead time is logistical: the facilities are operated by federal contractors and government officials should be present for a congressional inspection. "The current urgency of concerns about what is happening to children who have been separated from their parents makes it important to provide access as quickly as possible," he said.

Greenberg is currently a senior fellow at the Migration Policy Institute, a Washington think-tank focused on immigration.

Martin Levine, from the Miami suburb of West Kendall, came to the facility with a sign showing a Nazi guard pulling a child away from woman with a Jewish star, and saying "Nazis took children away. Trump inhumane."

Levine said he believed the policy of separating families was immoral.

"This is not a Democrat or Republican issue, because all of the former first ladies have found this policy despicable," Levine said.

Republican lawmakers from Miami-Dade County have condemned the policy of separating families crossing the U.S. border.

Florida's other senator, Republican Marco Rubio, said in a tweet Tuesday, "Let's change the law so we can hold families together while awaiting expedited hearings."