U.S. President Donald Trump on Thursday again threatened to seal the U.S.-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.

On the same day, Trump extended an immigration designation granting protected status to Liberians in the U.S. for an additional year, just three days before it was to expire, a rare move from a president who has taken a hardline stance on immigration.

Trump last year ordered an end to Deferred Enforced Departure (DED) status for Liberians, which was first granted to them during the presidency of his fellow Republican George W. Bush and enabled the immigrants to work and protected them from deportation. Advocates estimate that roughly 4,000 Liberians in the U.S. are protected by DED.

Trump had declared a one-year "wind-down" period, which set the final expiration date of the status as March 31. But on Thursday, Trump said he had changed his mind, winning unusual praise from some immigration advocates who have fought his policies in courts during the two years of his presidency.

"Upon further reflection and review, I have decided that it is in the foreign policy interest of the United States to extend the wind-down period for an additional 12 months, through March 30, 2020," Trump said in a memorandum to his secretaries of state and homeland security that was released by the White House.

"The reintegration of DED beneficiaries into Liberian civil and political life will be a complex task, and an unsuccessful transition could strain United States-Liberian relations and undermine Liberia's post-civil war strides toward democracy and political stability," he said.

Trump's administration has sought to eliminate similar programs for other nationalities, such as El Salvador, Haiti, Nicaragua, and Sudan. In such instances, the administration has argued that the domestic crises that resulted in those immigrants coming to the United States had long ago been resolved and that it was therefore time for them to return home.

Trump noted efforts in Congress to grant Liberians permanent legal status, and said his reprieve "will preserve the status quo while the Congress considers remedial legislation."

(Cover: Construction workers build a secondary border wall on February 22, 2019, in Otay Mesa, California. /VCG Photo)