(CNN) Democratic presidential hopeful Pete Buttigieg said Thursday that he was unaware when he said "all lives matter" in 2015 that the phrase was being used to undercut the Black Lives Matter movement.

Buttigieg spoke about his past comment after a speech at the National Action Network, a politically powerful organization founded by Rev. Al Sharpton. The phrase "all lives matter" has been used by critics of the Black Lives Matter movement to push back on the view that closer attention needs to be paid to the deaths of African American men and women at the hand of law enforcement.

"At that time, I was talking about a lot of issues around racial reconciliation in our community. What I did not understand at that time, was that phrase, just early into mid-2015, was coming to be viewed as a sort of counter slogan to Black Lives Matter," Buttigieg said. "And so, this statement, that seems very anodyne and something that nobody could be against, actually wound up being used to devalue what the Black Lives Matter movement was telling us."

He added: "That is the contribution of Black Lives Matter and it's a reason why, since learning about how that phrase was being used to push back on that activism, I've stopped using it in that context."

Buttigieg, who launched a presidential exploratory committee earlier this year, said "all lives matter" during his 2015 State of the City speech.

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