(AFP) President Donald Trump on Thursday again threatened to seal the US-Mexican border, claiming in a tweet that America’s southern neighbor is allowing illegal immigrants to cross unhindered. “May close the Southern Border!” the president wrote. “Mexico is doing NOTHING to help stop the flow of illegal immigrants to our Country. They are all talk and no action,” he said. “Likewise, Honduras, Guatemala and El Salvador have taken our money for years, and do Nothing. The Dems don’t care, such BAD laws.” The new threat to shut one of the world’s busiest borders, separating two countries with massive economic and cultural links, shows Trump is doubling down on his bid to make immigration a keystone of the gathering 2020 reelection campaign. Mexican President Andres Manuel Lopez Obrador rejected Trump’s criticism, telling journalists: “We are doing something on this issue.” “We are going to help in every way we can. We don’t in any way want a confrontation with the United States,” he said. But Lopez Obrador said a solution would depend on “fundamentally addressing the causes of migration.” Trump will likely highlight the issue when he hosts a campaign rally in Michigan later Thursday. On Wednesday, he referred to the need for more border walls to stop “people pouring in.” “Other countries stand there with machine-guns ready to fire. We can’t do that,” he told Fox News. “We are building massive, many, many miles of walls right now, and we are gearing up to do many more.”