Democrats in Congress accused Homeland Security Secretary Kirstjen Nielsen of lying amid intensifying outrage over a Trump administration policy requiring border agents to separate migrant children from their parents.

Several members of Congress called Nielsen out after she tweeted Sunday evening “we do not have a policy of separating families at the border.”

Rep. Hakeem Jeffries of New York called Nielsen a “stone cold liar.” Reps. Sheila Jackson Lee of Texas and Katherine Clark of Massachusetts also called Nielsen’s statement a “lie.”  California Rep. Eric Swalwell responded, “She knows we can check this, right?”

You are a stone cold liar. https://t.co/JXKb7ppwwt

— Hakeem Jeffries (@RepJeffries) June 18, 2018

The skirmish is an escalation of the dispute embroiling the Trump administration and members of Congress, including some Republicans, over its policy causing the separation of thousands of migrant families crossing the border in recent months.

Over the weekend, Lee and Jeffries were among a group of Democratic legislators who marched to immigrant processing and detention facilities across the country in protest of the new policy.

Lee joined Sens. Jeff Merkley of Oregon and Chris Van Hollen of Maryland and others at five facilities in Southern Texas, including two facilities separately holding adults and their children.

After a tour of the border facilities, Merkley and Van Hollen described in an interview with CNN “large pens of chainlink fences” holding about 1,500 migrant children and parents separated from their children after fleeing violence in Central America.

GOP Sen. Orrin Hatch condemned the policy Monday morning, saying in a statement that “a policy that leads to separating children from their families is wrong.”

HATCH on the child separation policy:

“A policy that leads to separating children from their families is wrong.” #utpol pic.twitter.com/yUU2YiRwkC

— Senator Hatch Office (@senorrinhatch) June 18, 2018

Rep. Will Hurd, who visited a temporary “tent city” for migrant children in Tornillo, Texas, said the separation of families “isn’t a Republican or a Democrat issue.”

“This is about a bad policy folks have created. I think what you saw in Tornillo was the manifestation of a terrible policy of separating kids from their families,” said Hurd, whose district includes a large swath along the U.S.-Mexico border.

Members of the New York and New Jersey congressional delegations, including Jeffries, visited a detention facility in New Jersey on Sunday morning, and Washington Rep. Pramila Jayapal met with immigrant mothers at a detention center near Seattle last week.

The conditions on the southern border are the result of a Trump administration policy that requires the Department of Justice to incarcerate all adults who cross the border to be criminally prosecuted for illegal entry.

While the policy does not explicitly state that children should be taken away from their parents, it effectively requires border agents to force families apart because federal law does not allow police to incarcerate immigrant children.

Previously, families crossing the border together would normally be released into the U.S. with an order to appear in immigration court.

The family separation issue has emerged as one of the most inflammatory issues in a legislative fight to pass immigration reform.

President Donald Trump is scheduled to meet Monday afternoon with two Republican senators who have the difficult task this week of balancing the president’s requests for funding for more immigration enforcement and a border wall with the realities of needing Democratic support to pass spending bills.

Senate Appropriations Chairman Richard C. Shelby of Alabama and Homeland Security Appropriations Subcommittee Chairwoman Shelley Moore Capito of West Virginia are scheduled to meet with President Donald Trump on Monday at the White House, a day before Capito’s panel marks up its draft fiscal 2019 spending bill.

Administration officials, including Nielsen, have said the administration’s policy on immigration detentions is necessary to fully enforce the nation’s laws. Some say the separation of families is a byproduct of properly enforcing the law, while others say it serves as a deterrent for other parents.

But the measure has been harshly denounced by activists, religious leaders, elected Democrats and Republicans alike.

Former first lady Laura Bush wrote a scathing op-ed in the Washington Post Sunday excoriating the policy as “cruel” and “immoral.”

“I live in a border state. I appreciate the need to enforce and protect our international boundaries, but this zero-tolerance policy is cruel,” Bush wrote. “It is immoral. And it breaks my heart.”

Nielsen is the latest administration official to be called out for misleading statements on the policy. Trump has repeatedly and falsely claimed that family separation is a Democratic policy required by law, rather than the byproduct of his own administration’s strict immigration enforcement policies.

“The Democrats are forcing the breakup of families at the Border with their horrible and cruel legislative agenda,” Trump tweeted Friday. “Any Immigration Bill MUST HAVE full funding for the Wall, end Catch & Release, Visa Lottery and Chain, and go to Merit Based Immigration. Go for it! WIN!”

Moderate Republican Sens. Susan Collins and Jeff Flake demanded answers about the treatment of migrant families in a Saturday letter to the DHS and the Department of Health and Human Services. The letter cited reports of mothers being separated from young children and infants, and asked whether any steps have been taken to make the process “less traumatic.”

Collins denounced the separations Sunday in an interview with CBS.

“What the administration has decided to do is to separate children from their parents to try to send a message that, if you cross the border with children, your children are going to be ripped away from you,” she said. “That is traumatizing to the children, who are innocent victims. And it is contrary to our values in this country.”

— Dean DeChiaro, Niels Lesniewski and Morgan Phillips contributed to this report.