CHIQUIMULA, Guatemala -- More than 2,000 Honduran migrants travelling en masse through Guatemala resumed their journey toward the United States on Wednesday as U.S. President Donald Trump sought to turn the caravan into a political issue three weeks before midterm elections.

A day after warning Central American governments they risk losing U.S. aid if they don't do something and saying that anyone entering the country illegally would be arrested and deported, Trump turned his sights on Democrats and urged Republican allies to campaign on border security.

"Hard to believe that with thousands of people from South of the Border, walking unimpeded toward our country in the form of large Caravans, that the Democrats won't approve legislation that will allow laws for the protection of our country. Great Midterm issue for Republicans!" Trump said in a Wednesday morning tweet.

"Republicans must make the horrendous, weak and outdated immigration laws, and the Border, a part of the Midterms!" he continued.

In Guatemala, the migrants rose early and many left without eating breakfast, bound for Zacapa, the next city on their route. Overcast skies and a light drizzle took the edge off the sweltering heat and humidity, making the trek more bearable.

Luis Navarreto, a 32-year-old migrant in the caravan, said he had read about Trump's threats regarding aid to his country but was undeterred.

"We are going to continue," Navarreto said. "It is God who decides here, we have no other option but to move ahead."

The migrants are fleeing widespread poverty and gangland violence in one of the world's most murderous countries, and many blamed Honduran President Juan Orlando Hernandez for what they called unlivable conditions back home.

"We are here because of Juan Orlando," said Nelson Zavala, a 36-year-old labourer who added that the last three days had essentially been sleepless ones.

The previous day the migrants advanced about 30 miles (40 kilometres) from the Honduras-Guatemala border to arrive at the city of Chiquimula.

That's a tiny portion of the almost 1,350 miles (2,200 kilometres) they'd have to travel to reach the closest U.S. border.

Some were able to hitch rides, packing the flatbeds of pickups and farm trucks, and even cargo holds of semis, while many more continued on foot with backpacks, strollers and Honduran flags. Hundreds advanced farther and faster than the main group to reach the Guatemalan capital, according to the Casa del Migrante shelter there.

Late Tuesday, the Office of the United Nations High Commissioner for Human Rights called on Honduras, Guatemala and Mexico to respect the rights and ensure the safety of the migrants travelling in the caravan.

The caravan has snowballed since about 160 migrants departed Friday from the Honduran city of San Pedro Sula, with many people joining spontaneously while carrying just a few belongings. Estimates of their numbers ranged up to 3,000.

Three weeks before the U.S. elections, the caravan was bound to draw Trump's ire. But he did not follow through on a similar threat to cut aid to Honduras in April over an earlier caravan, which eventually petered out in Mexico.

On Tuesday, Honduras' president accused unnamed "political groups" organizing the caravan based on lies in order to cause problems in Honduras.

"There are sectors that want to destabilize the country, but we will be decisive and we will not allow it," Hernandez told reporters.

Earlier the Foreign Ministry alleged that people had been lured to join the migration with "false promises" of a transit visa through Mexico and the opportunity to seek asylum in the United States.

In a joint statement Wednesday, Mexico's Foreign Relations and Interior Departments said that anyone in the caravan with travel documents and a proper visa will be allowed to enter, and anyone who wants to apply for refugee status can do so.

But the statement said all cases must be processed individually, suggesting that authorities have no intention of letting the migrants simply cross the border en masse without going through standard immigration procedures.

It warned that anyone who enters Mexico in an "irregular manner" faces detention and deportation.

None of the migrants The Associated Press spoke to on the road was carrying a passport. When agents in Guatemala near the Honduran border asked a crowd of them what documentation they were carrying, they held up national personal ID cards, which allow them to move through most countries in Central America -- but not Mexico, which requires foreigners to present a passport for entry.