US officials have said that around 680 undocumented migrants were detained in a major series of raids at food processing plants in the southeastern United States, 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.

TV footage showed lines of arrested migrants heading to buses parked outside the processing plants with their hands behind their backs under the watchful eyes of ICE agents.

Those who were not detained, including some children, stood by waving at their friends and relatives.

Several wiped away tears as they gave their farewells.

Some of those detained will be released for "humanitarian reasons" and required to appear in US immigration court, the US Attorney's Office of the Southern District of Mississippi, which partnered with ICE, said in a statement.

Others will be transported to an ICE facility in Jena, Louisiana, it 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.

"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.

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

"After speaking with the agents, I now know that they are identifying illegal immigrants, versus those who are legal, and rounding them up, putting them in buses and carrying them away," Canton Mayor William Truly told the local ABC News television affiliate.

Chokwe Antar Lumumba, the mayor of Mississippi state capital Jackson, took to social media to seek help for the migrants.

"I'm calling upon faith institutions in our community to become sanctuaries for our immigrant neighbours and protect them from potential harm," he wrote on his Facebook account.

"The ICE raids are both dehumanising and ineffective as a tactic for protecting citizens from potential harm."

In June, Mr 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".

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