Immigration advocates at the southern U.S. border say the Trump administration is lying when it says it's at "capacity" and can't process any more asylum seekers at the ports of entry where migrants can legally claim asylum.

On Twitter Monday night, U.S. Customs and Border Protection said "the processing system at CBP and our partner agencies has hit capacity."

The tweet was in response to news stories that 15 Honduran immigrants, including some children as young as five, were not allowed into the border station at Otay Mesa, California Friday night.

With the influx of Central American family units arriving at US ports of entry without proper documentation and crossing US borders illegally, the processing system at CBP and our partner agencies has hit capacity. https://t.co/iu8PZ9dFyi — CBP (@CBP) December 18, 2018

But Kara Lynum, a lawyer with American Immigration Lawyers Association who was held outside the Otay Mesa border station with the 15 Honduran immigrants Monday, said the station was not full. She said she knows this because the Border Patrol was able to find space for children crossing unaccompanied and for the family of Maria Meza, the Hondurans pictured running from tear gas at the border last month. A photo of the Meza family went viral on the internet.

"They magically had capacity to process the unaccompanied minors and to process the asylum seekers who were tear gassed in the picture," Lynum said.

A law enforcement official told NBC News that intake at ports of entry fluctuate, but that not all are at capacity. A CBP spokesperson declined to define "capacity" for NBC News.

Honduran asylum seekers enter the United States at the Otay Mesa port of entry on Dec. 18, 2018. Moises Castillo / AP

"CBP processes undocumented persons as expeditiously as possible without negating the agency's overall mission, or compromising the safety of individuals within our custody," said the spokesperson in response to NBC's inquiries.

Lindsay Toczylowski, the executive director of Immigrant Defenders Law Center, said officials have not been transparent when they have barred asylum seekers from entering the United States, including some of her clients.

"When they say they're at capacity at any given day it's unclear what they mean. They sent 5,000 troops to the border, a huge influx of resources, but the most vulnerable people arriving at our border — asylum seekers — are the ones they don't have any capacity to process? It doesn't make sense."