WASHINGTON – President Trump was back to warning Americans of an “invasion” of migrants coming up through Mexico.

On Sunday, Trump tweeted a warning from the mayor of Tijuana, Mexico, who the president loosely quoted as saying his city was ill-prepared to handle the anticipated 10,000 migrants moving north.

“No city in the world is prepared to receive this – if I’m allowed – this avalanche,” Tijuana Mayor Juan Manuel Gastelum said, according to the Associated Press . “It is a tsunami. There is concern among all citizens of Tijuana.”

Gastelum said there were already 2,750 migrants from the caravan who had arrived in the Mexican city across the border from San Diego, Calif.

He said he expected the influx of migrants to last six months, something Trump pointed out in his Sunday afternoon tweet.

“Likewise, the U.S. is ill-prepared for this invasion, and will not stand for it. They are causing crime and big problems in Mexico. Go home!” Trump wrote.

The president also pledged that migrants waiting for their asylum claims to be heard wouldn’t be allowed to go free in the U.S. until their court hearing.

“Catch and Release is an obsolete term. It is now Catch and Detain. Illegal Immigrants trying to come into the U.S.A., often proudly flying the flag of their nation as they ask for U.S. Asylum, will be detained or turned away,” Trump tweeted. “Dems must approve Border Security and Wall NOW!”

Earlier this month the Department of Justice and the Department of Homeland Security issued a new rule with the aim to make illegal border-crossers’ ineligible to make asylum claims.

Going forward, migrants claiming asylum will have to do so at U.S. points of entry.

U.S. border inspectors are processing about 100 asylum claims a day at Tijuana’s main crossing, according to the AP, which is creating a long wait.