Tecun Uman (Guatemala), Oct 30 (AP) Hundreds of Central Americans following in the footsteps of a thousands-strong migrant caravan making its way toward the US border crossed a river from Guatemala into Mexico on Monday, defying a heavy Federal Police presence deployed to patrol that country's southern frontier.

A low-flying police helicopter hovered overhead as the migrants waded in large groups through the Suchiate River's murky waters, apparently trying to use the downdraft from its rotors to discourage them.

Guatemala's Noti7 channel reported that one man drowned and aired video of a man dragging a seemingly lifeless body from the river.

Once on the Mexican side the migrants were surrounded and escorted by black-uniformed officers as sirens wailed. The standoff at the riverbank followed a more violent confrontation that occurred on the bridge over the river Sunday night, when migrants threw rocks and used sticks against Mexico police.

One migrant died from a head wound during the clash, but the cause was unclear.

Hundreds of miles up the road in southern Mexico, the first caravan of some 4,000 migrants resumed its advance, still at least 1,000 miles or farther from their goal of reaching the US as the Pentagon announced it would send 5,200 active-duty troops to "harden" the US-Mexico border. There are already more than 2,000 National Guard troops providing assistance at the border.

The caravan currently has about 4,000 people, but has been dwindling. Earlier this year, only about 200 from a caravan of some 1,000 migrants reached the Tijuana-San Diego frontier.

The Pentagon announcement comes as President Donald Trump has been focusing on the caravan to stir up his base a week before midterm elections.

On Monday he tweeted: "This is an invasion of our Country and our Military is waiting for you!"

Earlier in the day, members of the caravan strung out along the highway outside the city of Tapanatepec, some waiting for rides while others plodded toward their goal for the day: Niltepec, about 54 kilometres to the northwest. Federal Police patrols drove slowly alongside encouraging them to stay on the shoulder.

Victor Argueta, 54, of Santa Barbara, Honduras, said he and his wife had spent two nights sleeping on the international bridge between Tecun Uman, Guatemala, and Ciudad Hidalgo, Mexico, before eventually crossing the river on a raft.

"We came with the goal of wanting to improve our future for ourselves and for our family. We did not come with the intention of finding death on the road," Argueta said, reflecting on the news of the Honduran man's death the previous night. "Maybe that boy came with good intentions, perhaps with a young person's idea of supporting his family."

Sandra Rodriguez, 31, had heard about the incident because her husband's family lives in Tecun Uman. The couple from Guatemala City had joined the caravan in the border town and never considered someone could die on the bridge.

"I think they are risking much to cross to this side," Rodriguez said.

While catching rides from passing trucks was a largely impromptu affair in the first week of the caravan, it has now become more organized.

On Monday, more than 100 migrants lined up at a gas station parking lot to wait for rides.

Mayor Ramiro Nolasco of the town of Zanatepec said locals had organised a bus and several trucks to carry migrants, mainly women and children.

"We are helping our brothers from other countries with food, water, and transportation," Nolasco said. "It is going to be very little, compared to what they need."

At a checkpoint near the town, some migrants gathered to ask for help returning home to Honduras, the origin of the great majority of those in the caravan.

Exhausted from many days on the road, and disheartened by the many miles yet to go and misbehaviour by some fellow travellers, people have been dropping out from the caravan, which at its peak was estimated at more than 7,000.

The generosity shown by small towns and residents when the migrants first began trekking through southern Mexico has also lessened. At the last stop, few people came out to offer food, clothes and other items, said Hasiel Isamar Hernandez, a 28-year-old Honduran mother of three who has been with the caravan since it started in her hometown of San Pedro Sula.

"Of the friends that I have been with, all want to go back," Hernandez said, adding that many had blistered feet. For her, the last straw was when her husband told her that her 3-year-old daughter back home had stopped eating because she missed her mother.

Another Honduran, Teodozo Melendez, 31, was also waiting for a bus back home after fighting a fever for two days. His body ached.

"I thought it would be easier," Melendez said, lying on the ground.

Melendez's goal had been to join relatives living in Houston. His experience with the caravan had taught him one thing, he said: "The next time, I'm going to need a 'coyote,'" or smuggler.

The second group back at the Guatemalan frontier has been more unruly than the first that crossed. Guatemala's Interior Ministry said Guatemalan police officers were injured when the migrant group broke through border barriers on Guatemala's side of the bridge.

Mexico authorities said migrants attacked its agents with rocks, glass bottles and fireworks when they broke through a gate on the Mexican end but were pushed back, and some allegedly carried guns and firebombs. (AP) SMJ SMJ 10300434 NNNN.