President Donald Trump on Thursday threatened to close the U.S.-Mexico border, complaining that Mexico and several Central American countries were not doing enough to prevent the wave of migrants flooding north toward the U.S., an influx that border officials say is straining their resources.

“Mexico is doing NOTHING to help stop the flow of illegal immigrants to our Country. They are all talk and no action,” Trump tweeted. “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!”

The president’s threat and his renewed gripes about Mexico and so-called Northern Triangle governments come as the number of migrant families arrested at the border soared this month, putting immigration officials at what U.S. Customs and Border Protection Commissioner Kevin McAleenan called a “breaking point.”

McAleenan postponed an appearance before Congress on Wednesday to travel to the border, where he said the Border Patrol is on pace to arrest more than 55,000 family members in the month of March, a more than 500 percent increase over 2018 arrest levels.