US sending 5,200 troops to border as Trump warns migrant caravan to 'go back'

The US will send 5,200 active duty troops to "harden" the southern border against Central American migrants making their way through Mexico.

General Terrence O'Shaughnessy told reporters that 800 soldiers were en route on Monday and that the remainder of the troops would be at the southwest border by the end of the week.

Dubbed 'Operation Faithful Patriot', the move is an effort to help Customs and Border Protection at and near legal entry points.

Advanced helicopters will enable border protection agents to swoop down on migrants, he said.

"We're going to secure the border," Air Force Gen. Terrence

The deployment is in addition to more than 2,000 members of the National Guard are already providing assistance at the border.

Officials say the troops will provide "mission enhancing capabilities" and will be armed.

The move came after Donald Trump tweeted that the migrant caravan heading to the US was an "invasion".

He said: "Please go back, you will not be admitted to the United States unless you go through the legal process. This is an invasion of our Country and our Military is waiting for you!"

The military operation drew quick criticism.

"Sending active military forces to our southern border is not only a huge waste of taxpayer money, but an unnecessary course of action that will further terrorise and militarise our border communities," said Shaw Drake of the American Civil Liberties Union's border rights center at El Paso, Texas.

Migrants have turned down an offer by Mexico of benefits if they applied for refugee status in the country, vowing to continue their trek toward the US border.

The caravan of 3,500 has shrunk from a peak of about 7,200 migrants a week ago, but a second caravan of about 600 had formed and was clashing with federal police on a bridge from Guatemala to Mexico.