U.S. Homeland Security Secretary John Kelly on Wednesday told Guatemalans not to waste their money trying to get to the United States illegally, detailing how his department was cracking down on undocumented immigrants and would speed up deportations.

Speaking after he greeted a plane full of Guatemalans deported back from the United States, Kelly said U.S. authorities were not carrying out mass deportations of immigrants and would follow the law.

But he stressed that deportees would be returned their home countries more quickly than in the past.

"If you're a Guatemalan who is considering paying a great deal of money to a coyote (smuggler) to bring you to the United States, you'll be wasting your money," Kelly said at a Guatemalan Air Force base where 347 deportees earlier arrived on a flight.

"You will be returned very quickly," he said.

Kelly left Guatemala shortly after speaking to reporters, headed for Mexico where he and U.S. Secretary of State Rex Tillerson were expected to explain new U.S. immigration measures to their counterparts.

U.S. immigration courts have a large backlog of cases, meaning it can take years to deport some undocumented immigrants.

In recent years, tens of thousands of Central American children and families have moved to the United States, many seeking asylum from ruthless street gangs.

Kelly made his comments a day after his department said it planned to deport non-Mexican irregular immigrants to Mexico, regardless of their nationality.

The plan angered Mexican officials, with their lead negotiator and Foreign Minister Luis Videgaray saying there was no way Mexico would accept it.

(Writing by Christine Murray; Editing by Frank Jack Daniel and Jonathan Oatis)