Families crossing the US-Mexico border illegally surged to a "crisis" level in September as Trump administration policies failed to deter the inflow of migrants from Central America, officials said Tuesday.

The US Border Patrol apprehended 16,658 people who arrived in the country as families last month, some 900 more than August and nearly 12,000 more than September one year ago.

The surge was reported as President Donald Trump has called a caravan of some 7,000 migrants planning to traverse Mexico to the southern US border a "national emergency" and threatened to close the border completely to halt illegal immigration.

US Customs and Border Protection said for the full fiscal year that ended September 30, a total of 521,090 people without immigration documents have been apprehended or blocked at the Mexico border.

That was up by more than 105,000 from the previous year, Trump's first year in office when he declared that his tough crackdown on immigration was working.

Out of that, people arriving as families, many requesting asylum in the United States from the violence in their home countries, and as unaccompanied minors, has jumped as a percent of the total, to around 40 percent.

Speaking under condition of anonymity, a senior administration official called it a border crisis "that is unprecedented in our history."

The official said laws meant to protect the rights and safety of border crossers are also making it impossible to return people to their home countries and forcing the government to process their asylum claims and release them into the United States.

"The cost on society is enormous," the official said.

"We are apprehending these aliens. If we could return them, there would be no crisis.

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