US officials said that some 680 undocumented migrants have been detained in raids at food processing plants in the southeastern United States, as part of President Donald Trump's announced crackdown on illegal immigration.

Most of those detained by Immigration and Customs Enforcement (ICE) agents were Hispanic migrants, officials said.

"Special agents executed administrative and criminal search warrants resulting in the detention of approximately 680 illegal aliens," said Mike Hurst, US Attorney for the Southern District of Mississippi.

"They have to follow our laws, they have to abide by our rules, they have to come here legally or they shouldn't come here at all," Mr Hurst said at a news conference.

The US attorney did not spare the employers. 

Matthew Albence, the interim ICE head, said the raids were the result of a year-long investigation

"To those who use illegal aliens for competitive advantage or to make a quick buck, we have something to say to you: If we find that you have violated federal criminal law, we're coming after you," he said.

Matthew Albence, the interim ICE head, said the raids were the result of a year-long investigation.

He said that the children of detained parents will be sent to live with relatives or other families.

Some of the migrants will be released with electronic ankle monitors as they await a court hearing.

ICE agents raided food processing plants in the towns of Morton, Carthage, Canton, Pelahatchie, Sebastopol and Bay Springs, all in the state of Mississippi, officials said.

In June, President Trump tweeted that ICE "will begin the process of removing the millions of illegal aliens who have illicitly found their way into the United States."

Mr Trump has also tweeted several times about an alleged "invasion" of people crossing the southern border into the United States.