US President Donald Trump has confirmed on Friday that agents of the Immigration and Customs Enforcement agency will launch raids across the country this weekend to round up thousands of undocumented migrants for deportation.

"They came in illegally," he told reporters at the White House. 

"It starts on Sunday and they're going to take people out and they're going to bring them back to their countries," Trump said.

"Or they're going to take criminals out, put them in prison, or put them in prison in the countries they came from," he added.

While the focus will be on removing criminals, Trump said the raids would also target "people that came into our country, not through a process, that just walked over a line. They have to leave."

The ICE raids are expected to take place in 10 major cities, pursuing people for whom courts have already issued removal orders, according to media reports.

'Brutal action' 

They could potentially target families who have been inside the United States for many years, with homes, businesses and US-born children, the reports said.

Migrant communities and immigration and rights activists around the country were girding for the raids.

Migrants were being told to not open their doors to ICE agents if they do not have search or arrest warrants, to record their encounters with agents, and to call immigration attorneys for help.

Democrats warned the Trump administration on Thursday about breaking up long-resident families with members who are inside the country legally.

House leader Nancy Pelosi called the ICE plan "heartless" and said Sundays are when many Hispanic immigrant families are in church.

"These families are hardworking members of our communities and our country. This brutal action will terrorize children and tear families apart," she told reporters.

"Many of these families are mixed-status families," she added, referring to families who include members in the United States legally and illegally, such as migrants with children born inside the country.

According to the Pew Research Center, there are about 10.5 million undocumented migrants in the United States, and two-thirds have been in the country for more than 10 years.

'Outstanding job'

Ken Cuccinelli, acting director of US Citizenship and Immigration Services, said on Wednesday that ICE has removal orders for some one million migrants, but added that it has nowhere near the manpower or facilities to arrest and deport that many.

Trump meanwhile praised the Mexican government for helping to crack down on the number of migrants passing through Mexico to cross the southern US border.

He noted that the number of migrants detained while entering the United States had fallen in June, after he threatened punitive tariffs on imports from Mexico that could have crippled its economy.

On Wednesday, the Department of Homeland Security said 104,344 migrants were detained after crossing the border in June, down 28 percent from May's 13-year record high but still an extremely high figure, some 60,000 more than the same month last year.

"Mexico has done an outstanding job so far," Trump said, adding that the country had sent 21,000 soldiers to its southern and northern borders to stem the flow of migrants, most of whom are from Guatemala, El Salvador and Honduras.

"Yes, they maybe did it because of tariffs, but they're doing a great job and I appreciate it," Trump said