An "unprecedented" number of migrants were intercepted illegally crossing the US-Mexico border in July, the head of America's Homeland Security department has said.

Border guards recorded more than 200,000 instances of people attempting to enter the United States during the month, according to US Customs and Border Protection.

"We are encountering an unprecedented number of migrants in between the ports of entry at our southern border," Homeland Security Secretary Alejandro Mayorkas told a press conference in Texas Thursday.

"The situation at the border is one of the toughest challenges we face. It is complicated, changing and involves vulnerable people at a time of a global pandemic."

A Guatemalan migrant attempts to enter the United States undetected in New Mexico
A Guatemalan migrant attempts to enter the United States undetected in New Mexico AFP / PAUL RATJE

Migrant arrivals to the United States declined markedly during the initial phase of the Covid-19 pandemic but began to pick up later in 2020, before surging since President Joe Biden took office.

The latest figures demonstrate a deviation from seasonal trends. Punishingly hot summer weather usually sees the number of migrants attempting to cross decrease.

Border authorities also said the number of unaccompanied children increased 24 percent in July, to just under 19,000, from the previous month.

A significant number of expulsions during the pandemic has also contributed to "a larger-than-usual number of migrants making multiple border crossing attempts."