President Trump speaks before a ceremony on Wednesday at the White House. (Jabin Botsford/The Washington Post)

President Trump on Thursday renewed his threat to close the southern border, claiming in a morning tweet that Mexico is doing “NOTHING” to stop the flow of undocumented immigrants into the United States.

“They are all talk and no action,” Trump said in his tweet. “Likewise, Honduras, Guatemala and El Salvador have taken our money for years, and do Nothing. The Dems don’t care, such BAD laws. May close the Southern Border!”

Trump’s tweet comes a day after the nation’s top border official warned that the U.S. immigration enforcement system along the nation’s southern boundary is at “the breaking point” and said that authorities are having to release migrants into the country after background checks because of a crush of asylum-seeking families with children.

Kevin McAleenan, the commissioner of U.S. Customs and Border Protection, said that for the first time in more than a decade, his agency is “reluctantly” performing direct releases of migrants, meaning they are not turned over to Immigration and Customs Enforcement, they are not detained, they are not given ankle bracelets to track their movements and they are allowed to leave with just a notice to appear in court at a later date.

Trump has threatened to close the border before but not followed through.

In November, in the heat of a battle with Congress over funding for his long-promised border wall, Trump wrote on Twitter that “we will close the Border permanently if need be.”

Trump has since declared a national emergency at the border as a way to spend more on barriers than Congress has authorized.

Nick Miroff and Maria Sacchetti contributed to this report.