The number of US border apprehensions has dropped for the eighth straight month, following crackdowns by the Trump administration that include forcing asylum seekers back over the US-Mexico border to wait out their claims, a Homeland Security official said Monday.

The official said the number of encounters with border officials over the past four months was 165,000. A year earlier during the same time it was about 242,000. The official spoke to The Associated Press on condition of anonymity because the official results have not been released.

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The tally for January was about 36,000, including apprehensions of people crossing irregularly and migrants who were declared inadmissible by border officers at a port of entry. It was a 10 percent decline from December.

The steep decline will almost certainly figure heavily in President Donald Trump's State of the Union address on Tuesday night. Trump has made cracking down on immigration a signature issue. He has described asylum seekers and other border crossers as con artists who "scam" the system, and derided immigrants from Mexico as "bad hombres".

Trump uses the monthly border tallies as a benchmark to determine how his policies are working, railing against Homeland Security officials when the numbers are up. The number of people crossing the border traditionally declines when it is hot outside - but the winter months often see creeping increases.

The monthly tally is down almost 75 percent from the peak last May, when there were more than 144,000 encounters with migrants, the large majority families from Central America who are not easily returned over the border.

The immigration system was vastly strained last spring, with migrants and asylum seekers crammed for weeks into small border stations not meant to hold people beyond a few days. News of the conditions in the border stations, coupled with migrant deaths, promoted massive outrage and pushed Congress into emergency funding to help ease the crush.

The reduction comes at a cost. More than 55,000 asylum seekers, including families and pregnant women, have been sent over the border to Mexico to wait out their asylum cases and have faced sickness and squalid conditions in makeshift camps, plus assault and kidnapping by cartels that patrol the borderlands.

Mexican National Guardsmen blocking the passage of migrants on the highway leading to Tapachula, Mexico [Marco Ugarte/AP Photo]

Mexico has also stepped up its own border enforcement, making it clear that caravans that once travelled through its territory are no longer allowed to do so, following intense pressure and threatened trade tariffs from Washington last year.

And United States policy now essentially bans anyone from claiming asylum if they crossed through another country first. Officials are also now sending asylum seekers to Central American nations as part of border security agreements with Guatemala, Honduras and El Salvador.

Despite the nosedive at the border, asylum seekers are still signing up on a waiting list to enter the US at an official crossing in San Luis, Arizona. US Customs and Border Protection calls the Mexican shelter that manages the list to find out how many asylum claims it will process each day. The shelter estimates the wait at three to four months.