Kamala Harris has made her first trip as vice-president to the nation's southern border as the White House grapples with political pressure over a growing migrant crisis.
In El Paso, Texas, Ms Harris called for an end to "finger-pointing". She also criticised ex-President Donald Trump.
One of her fellow Democrats said this administration's handling of the border situation made the party look "weak".
A record level of undocumented migrants have arrived at the border this year.
The numbers of asylum seekers and economic migrants fleeing poverty, corruption and gang violence in Latin America are only expected to grow during the summer months. This year US border agents have also separately apprehended two Yemeni men who were on a terror watchlist.
In Friday's brief trip, Ms Harris visited immigration facilities and met girl migrants aged 9-16 not far from border.
"They were asking me questions: 'How do you become the first woman vice-president?'" Ms Harris said.
"It also reminds me of the fact that this issue cannot be reduced to a political issue. We're talking about children, we're talking about families, we are talking about suffering."
She called for an end to political "finger-pointing" and "infighting". During the trip she also said: "It is here in El Paso that the previous administration's child separation policy was unveiled," in a reference to Mr Trump's splitting up of migrant families.
Critics said Ms Harris should have visited a tent complex at nearby Fort Bliss, where migrant children are being held.
A BBC investigation of the Fort Bliss detention centre found reports of sexual abuse, Covid and lice outbreaks, hungry children being served undercooked meat and sandstorms engulfing the desert tent camps where the young people are being held.
Ms Harris' motorcade was greeted by pro-Trump protesters waving flags. One demonstrator's sign said: "How many little girls need to be raped for this to be a crisis?"
US President Joe Biden's administration said on Friday it would investigate conditions at the site. It also announced Health Secretary Xavier Becerra would travel there next week.
Mr Biden tasked Ms Harris back in March with addressing the root causes of immigration. This month she visited Guatemala and Mexico, where she asked that potential migrants not make the trek to the US.
But the vice-president had until now repeatedly deflected questions about whether she ought to visit the US-Mexico border, dismissing such a trip as a "grand gesture".
The White House announced her visit this week after Mr Trump, a Republican, said he would head to the border on 25 June.
Ms Harris denied on Friday that she was only going because of Mr Trump, though her travel caught White House reporters unawares since they are usually briefed a week ahead of such trips for planning purposes.
"I said back in March I was going to come to the border, so this is not a new plan," the vice-president told reporters after landing in Texas on Friday.
Ms Harris faced inevitable criticism from Republicans like Texas Senator Ted Cruz, who dismissed her visit as "a glorified photo-op".
But she was also upbraided by a fellow Democrat, congressman Henry Cuellar, whose constituency straddles part of the southern border.
"The administration is making Democrats look weak," he told the Associated Press news agency.
"I've heard, from Democrats and Republicans in my area, what the heck is going on with this administration?"
US Customs and Border Protection said it caught 180,034 migrants in May, the biggest monthly total since 2000, and up slightly from the previous two months.
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