Washington (AFP) - Back Wednesday from her debut international mission as US vice president, Kamala Harris caught flak from Republican critics accusing her of not taking the migration crisis seriously enough, but also from vocal progressives frustrated about border policy.

The two-sided pile-on reflects the protracted and difficult nature of the US immigration problem, which has vexed administrations for decades.

Harris' trip to Guatemala and Mexico, during which she appeared to have flip responses to questions about why she has yet to visit the southern US border for an on-the-ground assessment, marks her biggest stumble yet since she became President Joe Biden's deputy.

Texas Republican Senator John Cornyn described her trip as a "missed opportunity." 

Harris "is avoiding the point of failure in our immigration system, which is at the border," Cornyn told AFP Wednesday in the US Capitol.

Less than a decade ago then-president Barack Obama faced a humanitarian crisis at the border, and struggled to come up with an effective response to handle thousands of people -- including unaccompanied minors -- fleeing violence and poverty in their home countries.

His successor Donald Trump famously vowed to build a border wall, and was fiercely criticized for controversial detentions.

Now its Biden's turn to handle undocumented immigration, and critics smell blood -- particularly Harris' explanation as to why she has yet to visit the US-Mexico border since the president tasked her with the immigration portfolio.

In Guatemala Harris said she would remain focused on addressing the root causes of illegal migration -- poverty and crime -- rather than the "grand gestures" of a hypothetical border visit.

In an interview with NBC News things grew tense. When Harris said "we've been to the border," the reporter noted that she had yet to visit in-person as vice president.

"And I haven't been to Europe," she retorted, before breaking into a nervous laugh. The exchange brought fierce criticism from Republicans.

'Opposite' messaging

Detentions of undocumented migrants along the US-Mexico border, including unaccompanied minors, hit a 15-year high in April. 

Nearly 180,000 people were intercepted -- more than 80 percent of them coming from Mexico or the so-called Northern Triangle of Guatemala, Honduras and El Salvador.

"Our recently secured Southern Border is now worse than ever before," Trump said in a statement Wednesday.

Under his presidency, anyone crossing the US border with Mexico illegally was criminally prosecuted and parents were placed in detention centers separate from their children.

Of the 4,000 migrant children separated during that period, more than 2,000 have yet to be reunited with their relatives, according to US authorities.

Harris arrived Sunday in Guatemala, where she levelled a firm warning to would-be migrants: "Do not come" to the United States.

But she also brought the Biden administration's pledge of a more humane immigration policy -- in stark contrast with Trump's "zero tolerance" approach, favored by Republicans.

"Everything Joe and Kamala have done has sent exactly the opposite message" from Harris' "do not come" remark, Republican Texas Senator Ted Cruz told Fox News on Tuesday.

"We've seen in the five months they've been in office, an absolute disaster unfolding at the border" with no willingness to fix the problem, Cruz said.

He noted the surge in undocumented arrivals, Biden's ending of Trump's "Remain in Mexico" program, and the refusal to stop the practice of catch and release for low-risk migrants allowed to stay in the United States pending immigration hearings.

Harris "literally flew twice over our southern border," another Republican, Senator James Lankford, told AFP.

The vice president's "do not come" remarks also antagonized those on her party's left flank.

"Disappointing" is how House Democrat Alexandria Ocasio-Cortez described Harris' warning.

"This whole 'stay there and die' approach is not how our country will promote a more humane and just immigration system," Congresswoman Rashida Tlaib tweeted.

The White House meanwhile offered Harris its languid support, saying her trip to Central America was "exactly what the president asked her to do."

Democrat Tim Kaine, who chairs the Senate Western Hemisphere Subcommittee on Foreign Relations, downplayed criticism of Harris as unfair and opportunistic.

"I imagine she knows a lot more about the border than some of the folks that are criticizing her," he said.