By Nelson Renteria and Delphine Schrank
SONSONATE, El Salvador/TAPANATEPEC, Mexico (Reuters) - A new group of migrants bound for the United States set off from El Salvador on Sunday, following thousands of other Central Americans fleeing poverty and violence who have taken similar journeys in recent weeks.
The group of more than 300 Salvadorans left the capital San Salvador on Sunday. A larger group of mostly Hondurans, estimated to number at least 3,500, who left their country in mid-October and are now in southern Mexico, has become a key issue in U.S. congressional elections.
A third group have been moving through Guatemala, at one point numbering more than 1,000 people before beginning to fragment. Hundreds of that group broke through a Guatemala border gate in Tecun Uman, and on Sunday afternoon were on the bridge into Mexico.
U.S. President Donald Trump and his fellow Republicans have sought to make immigration a major issue ahead of Nov. 6 elections, in which the party is battling to keep control of Congress.
Homeland Security Secretary Kirstjen Nielsen on "Fox News Sunday" said Trump was determined to use every authority he had to stop immigrants from crossing the border illegally.
"We have a crisis at the border right now ... This caravan is one iteration of that but frankly we essentially see caravans every day with these numbers," she said.
"I think what the president is making clear is every possible action, authority, executive program, is on the table to consider, to ensure that it is clear that there is a right and legal way to come to this country and no other ways will be tolerated."
Trump has threatened to shut down the border with Mexico and last week said he would send troops. On Friday, Defense Secretary Jim Mattis authorized the use of troops and other military resources at the U.S.-Mexico border.
By Sunday afternoon, dozens of the Salvadorans had arrived at the border with Guatemala and were having their documents checked, having walked and hitched rides in pickups and on buses from the capital.
They organised using social networks like Facebook and WhatsApp over the last couple of weeks, inspired by the larger group in Mexico.
Salvadoran police travelled with the group, who carried backpacks and water bottles and protected themselves from the hot sun with hats.
Several migrants, gathered by the capital's 'Savior of the World' statue before leaving, said they were headed to the United States.
El Salvador's left-wing government said it had solidarity with the migrants and respected their right to mobilize, but urged them not to risk their lives on the way.
In Mexico, the original group of Hondurans, exhausted by constant travel in blistering heat, spent Sunday resting up in the town of Tapanatepec, Oaxaca, planning to head north at 3am on Monday.
"It's far ... the farthest yet," said Honduran Bayron Baca, 26, pulling open a map that Red Cross volunteers had given him in a medical tent.
Dozens took dips in a nearby river to refresh from the trek, which has covered an average 30 miles (48 km) a day.
An estimated 2,300 children were travelling with the migrant caravan, UNICEF said in a statement, adding that they needed protection and access to essential services like healthcare, clean water and sanitation.
Eduardo Grajales, a Red Cross volunteer in Arriaga, Mexico, attending to migrants on Friday night, said the worst case his colleagues had seen that day was of a baby so badly sunburned from the tropical heat, he had to be hospitalized.
(Reporting by Nelson Renteria and Delphine Schrank, additional reporting by Carlos Rawlins and Doina Chiacu; Writing by Christine Murray; Editing by Andrea Ricci and Rosalba O'Brien)