The Senate has delayed a vote on Donald Trump’s pick to lead the nations's Immigration and Customs Enforcement agency.

The move comes amid scenes of high drama on the US-Mexico border where border guards fired tear gas at hundreds of migrants who pushed past Mexican police and ran towards the fence.

Yet politicians on Capitol Hill, said the delay was to allow the Senate Homeland Security committee do more “due diligence” on Ronald Vitiello, following his 15 November confirmation hearing. Senators expressed concerns after receiving a from agency staff, Republican Senator Ron Johnson said.

Unions representing ICE employees, said in a letter to the committee, Mr Vitiello "has only been with ICE for a short period of time, and while this letter neither supports nor opposes his nomination to be our director, we are aware of several matters that give us serious concern about him”.

Mr Vitiello is currently the acting commissioner of the agency, but requires Senate confirmation to assume the role in full.

Among the concerns, is Mr Vitiello’s social media activity, such as his sharing of images comparing Mr Trump to the cartoon character Dennis the Menace, the Washington Post reported.

The tweet, posted on a now-private account, was sent on 4 March 2016 with the text “This I can tell you! 100%”, accompanying an image of the president and the character making the same impression.

Shepard Smith on immigration: 'There is no invasion. No one is coming to get you. There is nothing at all to worry about.'

He also came under fire during his confirmation hearing, about a 2015 tweet in which he suggested alternate names for the Democratic Party could be "liberalcratic party or the NeoKlanist party".

Mr Vitiello started in his current position in July 2018, just after calls for abolishing ICE were made by several members of congress in light of the controversial family separation policy instituted by the Trump administration.

Despite an executive order ending the practice of arresting and detaining parents and guardians in separate facilities from children, during his confirmation hearing Mr Vitiello did not rule out the possibility it could be reinstituted.

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“We’ll get less people bringing their children,” he said, adding: “It is an option.”

The administration had floated the idea of giving migrant families who are seeking asylum 20 days to make the choice between staying in detention with their children until deportation hearing or letting their children be kept in a separate facility so relatives already in the US can come claim custody of them.

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During the hearing, Mr Vitiello did note the administration was not prepared for the near-universal outcry over family separation.

He said they “never contemplated on having the systems work backwards. Nobody in the discussions that I was involved in were contemplating that these people would be separated forever”.

“We’d like to be in a place where lots of people didn’t bring their kids to the border and try to cross illegally, but that’s the situation we’re faced with now,” he added.