WASHINGTON: 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.