President Donald Trump said Monday the US will start cutting aid to Guatemala, Honduras and El Salvador as a caravan of thousands of mostly Honduran migrants rolled on regardless toward the US border. Trump kept up his almost-daily Twitter attacks on the approaching caravan, calling it a national emergency and said he had alerted the US border patrol and military. "We will now begin cutting off, or substantially reducing, the massive foreign aid" that the United States provides to the three Central American countries, he said. Mexican authorities had managed to block the "caravan" of migrants on a border bridge between Mexico and Guatemala, but many later crossed the river below in makeshift rafts before marching north. The caravan of around 3,000 migrants was heading Monday to the town of Huixtla, around 40 kilometres further on from Tapachula in Chiapas State where they slept Sunday night. "Sadly, it looks like Mexico's Police and Military are unable to stop the Caravan heading to the Southern Border of the United States. Criminals and unknown Middle Easterners are mixed in," Trump said in one tweet. "I have alerted Border Patrol and Military that this is a National Emergy (sic). Must change laws!" Activists say the journey through of at least 3,000 kilometres through Mexico to the US border could take a month. "We are well aware that this country (Mexico) didn't receive us as we expected, and they can return us to Honduras, and we also know there are drug traffickers who kidnap and kill migrants," Juan Flores, one of those migrants, told AFP. "But we live with more fear in our country, so we carry on forward," he added. US Secretary of State Mike Pompeo also warned that the migrants "may be victimised by human smugglers or others who would exploit them." "We also are deeply concerned by the violence provoked by some members of the group, as well as the apparent political motivation of some organizers of the caravan," Pompeo said in a statement. About a thousand migrants, including women and children, were still stranded on a border bridge hoping to enter Mexico legally via Guatemala. Mexican authorities insisted those on the bridge would have to file asylum claims one at a time in order to enter the country. And another separate group of about 1,000 Hondurans started their own march across Guatemala, headed for Mexico and then the United States. The group of men, women and children gathered in Esquipulas before setting out on foot. "No one is going to stop us, after all we've gone through," said 21-year-old Aaron Juarez, who was accompanied by his wife and baby and was walking with difficulty because of an injury. Honduran farmer Edwin Geovanni Enamorado said he was forced to leave his country because of intimidation by racketeering gangs. "We are tired, but very happy, we are united and strong," he said. Britany Hernandez added: "We have sunburn. We have blisters. But we got here. Our strength is greater than Trump's threats." Mexico's President-elect Andres Manuel Lopez Obrador called for fair treatment of the migrants. "We don't want them to face what (Mexicans) face when they need to look for work in the United States," he said on Twitter. The caravan left San Pedro Sula in northern Honduras more than a week ago. It has comprised between 3,000 and 5,000 people at various times as it moved through Guatemala, according to various sources. Guatemalan President Jimmy Morales said more than 5,000 migrants had entered Guatemala from Honduras, but that some 2,000 had since returned home on a fleet of government-chartered buses On Saturday, Mexican authorities opened the border for women and children on the overcrowded bridge, taking them to a shelter in Tapachula, about 40 kilometers from the border. Plenty of migrants, however, are sleeping in the streets for fear that immigration officials could arrest them if they are in a shelter. Around 900 migrants - tired of waiting on the bridge - resorted to crossing the Suchiate River below on makeshift rafts and police did not intervene as they clambered up the muddy riverbank on the Mexican side on Saturday. Morales and his Honduran counterpart Juan Orlando Hernandez said after meeting that the march was "violating the borders and the good faith of the states." The Honduran president acknowledged that social problems were a contributory factor. "Without a doubt, we have a lot to do so that our people can have opportunities in their communities," he said. The migrants are generally fleeing poverty and insecurity in Honduras, where powerful street gangs rule their turf with brutal violence. With a homicide rate of 43 per 100,000 citizens, Honduras is one of the most violent countries in the world, according to a Honduran university study.