The number of people arrested crossing the Mexico border into the US has fallen to the lowest level in 17 years.
There were fewer than 17,000 arrests of undocumented migrants in March, the least since 2000, according to US Customs and Border Protection.
The US government began accepting bids on Tuesday from contractors to build a prototype for a border wall.
Mr Kelly, a retired Marine general, is testifying about the figures to the Senate Homeland Security Committee.
“We’ve seen an absolutely amazing drop in the number of migrants coming out of Central America that are taking that terribly dangerous route from Central America to the United States,” he told the panel.
“March marks the fifth straight month of decline and is estimated to be approximately 71% lower than the December 2016 total – 58,478,” Mr Kelly said.
“But while this recent decline in illegal migration is good news, we must ensure that the security of our southern border remains a priority to protect the nation from terrorists and other criminals.”
In February, 23,589 immigrants were apprehended at the border, according to data by US Customs and Border Protection.
Senator John McCain asked if Mr Kelly’s agency was receiving help from Mexican officials.
Mr Kelly said the US was receiving “a huge amount of co-operation from the Mexicans”.
He cited “very, very good relationships with the Mexicans, both on their southern border where they stopped 160,000 illegal immigrants from Central America last year”.
Mr Trump, a former real estate mogul, has pledged to build a “big, beautiful wall” along the 2,000 mile (3,200km) border.
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