Donald Trump’s purge of top officials at the Department of Homeland Security (DHS) has alarmed congressional Republicans, who warn the president’s immigration shake-up is creating a “growing leadership void”.

The president’s allies fear further sackings are on the cards following the ousting of a string of senior figures including homeland security secretary Kirstjen Nielsen.

The overhaul has been fuelled by Mr Trump's frustration with the increasing number of migrants at the US-Mexico border and was reportedly orchestrated by his adviser Stephen Miller, the impetus behind some of the administration's most controversial policies.

Ms Nielsen submitted her resignation on Sunday after meeting with the president at the White House. Three days earlier, the administration withdrew the nomination of Ron Vitiello to lead Immigration and Customs Enforcement.

The head of US Citizenship and Immigration Services, Lee Francis Cissna, and homeland security general counsel John M Mitnick are also expected to be forced out of their positions, officials told Associated Press.

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Other long-time civil servants in agency posts also face the chop, said the officials, who spoke to on condition of anonymity because they were not authorised to speak publicly.

Court challenges, immigration laws and Mr Trump’s own advisers have blocked several of his proposals as his re-election campaign looms.

But Republicans said a new immigration team was likely to face the same obstacles.

“It’s a mess,” Texas senator John Cornyn told Politico. “Strikes me as just a frustration of not being able to solve a problem. Honestly, it wasn’t Secretary Nielsen’s fault. It wasn’t for lack of effort on her part. I don’t know if there’s anybody who’s going to be able to do more.”

Wisconsin Republican Ron Johnson, chair of the Homeland Security Committee, said: "I am concerned with a growing leadership void within the department tasked with addressing some of the most significant problems facing the nation."

Democrats have also voiced alarm over the dismissals.

"The purge of senior leadership at the Department of Homeland Security is unprecedented and a threat to our national security," said Dianne Feinstein, of California. "President Trump is trying to remake DHS into his own personal anti-immigration agency."

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Beyond changing senior personnel, Mr Trump is also considering resuming the separation of migrant parents from their children at the border, a policy which provoked outrage before it was scrapped last year. That could mean he would have to violate his own executive order and possibly run afoul of a federal judge overseeing reunifications.