(CNN) Joe Biden on Thursday lamented the deaths of six migrant children while in US custody, saying it's "not who we are" in a statement that mirrors how the former vice president has framed much of his 2020 presidential campaign.

"Over the last year, six children have tragically died in US custody at the border. It's unacceptable. It's not who we are. And silence is complicity," Biden, the front-runner among the Democratic presidential candidates, tweeted. "It's on all of us to stand up and speak out. America is a nation of immigrants. We must guarantee everyone's treated with dignity."

On Wednesday, a Department of Health and Human Services spokesman said a 10-year-old migrant child died while in the care of the department's Office of Refugee Resettlement in September last year.

The death was not reported publicly for nearly eight months and marks the sixth migrant child known to have recently died while in US custody after traveling to the US and being apprehended by federal authorities.

Biden's comments on the deaths of migrant children echo a common refrain of his presidential campaign, warning that the nation's values are at stake under the current administration.

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