A group of doctors seeking to vaccinate migrants detained by President Donald Trump's Customs and Border Patrol were denied at the gates of the Chula Vista Border Patrol Station in San Ysidro, San Diego Monday.

"Refusing to vaccinate is inhumane," tweeted Immigrant Families Together director Julie Schwietert Collazo. "The agency is willingly putting lives at risk of death."

It was the latest example of what critics of the administration call the "intentional cruelty" of Trump's immigration policies.

"People are needlessly suffering and dying," said Dr. Marie DeLuca, one of the physicians who was turned away. "You can't lock people up in inhumane conditions, watch them get sick, and then refuse them access to medical care."

Doctors outside CBP San Ysidro with 120 doses of flu vaccines are asking government to allow them to vaccinate migrant children in detention for free.

Trump administration refuses to vaccinate children in CBP custody even after 3 children died of the flu.https://t.co/SHRd37wcQu https://t.co/pfSuQmuEFr — F A R R A H F A Z A L (@FarrahFazal) December 9, 2019

The doctors were supported by a coalition of immigration advocates, including the groups Doctors for Camp Closures, Families Belong Together, and Never Again Action. Monday's action marks the beginning of a planned week of action by the doctors and others at Chula Vista.

WATCH: Immigrants are dying from the flu. These doctors have been asking for permission to deliver free vaccines for months, and CBP just ignores them. So now they’re at the gates of a detention center in person, living out their oath to provide care. Follow @Doc4CampClosure. pic.twitter.com/omlW8nOFus — Never Again Action (@NeverAgainActn) December 9, 2019

In a statement, Families Belong Together chair Jess Morales Rocketto said that recent reporting from ProPublica on the death from flu and border patrol inaction of an immigrant child in detention gave the protest added urgency.

"This administration's cruel immigration agenda is intentionally barring life-saving medical care from children by refusing to provide vaccines during flu season," said Rocketto. "Earlier this year, as immigration authorities sat by, 16-year-old Carlos Gregorio Hernandez Vasquez died from the flu on the floor of his concrete jail cell."

Our statement from Families Belong Together Chairwoman @JessLivMo on today’s action and our demand to let doctors treat children. #FluVaccines4Migrants pic.twitter.com/KxwmCm0hCz — Families Belong Together (@fams2gether) December 9, 2019

The flu deaths and detention conditions are just the precursor to a greater horror, said Never Again Action LA spokesperson Jamie Goodman.

"We recognize these conditions from what many of our ancestors narrowly escaped," Goodman said. "Genocide doesn't start with death camps, it starts with what CBP concentration camps look like right now."