WASHINGTON - U.S. President Donald Trump on Sunday claimed Sunday that Mexico 'for many years' has not been cooperative to curb the surge of migrants traveling through it to reach the United States, but believes a new agreement will alleviate the problem.

The U.S. leader warned, however, that 'if for some unknown reason' Mexico does not stanch the flow of Central American migrants heading north to the U.S., 'we can always go back to our previous, very profitable' imposition of tariffs on Mexican exports sent to the United States. 'But I don't believe that will be necessary,' he added.

A deal announced Friday calls for Mexico to dispatch 6,000 troops to its border with Guatemala to halt the flow of migrants, while the U.S. gained new authority to force asylum seekers to stay in Mexico while their legal cases in the U.S. are pending. Trump said there is one particular provision of the pact that has yet to be disclosed but will be announced 'at the appropriate time.'

'There is now going to be great cooperation between Mexico & the USA, something that didn't exist for decades,' he said on Twitter.

'Now I have full confidence, especially after speaking to their President (Andres Manuel Lopez Obrador) yesterday, that they will be very cooperative and want to get the job properly done,' Trump said.

He dismissed an account in The New York Times as 'another false report' that key parts of the deal had been reached in December. He contended that the 'failing' newspaper and the 'ratings challenged' CNN television network 'will do anything possible to see our Country fail! They are truly The Enemy of the People!'

Trump's acting Homeland Security secretary, Kevin McAleenan, told Fox News Sunday "There's a mechanism to make sure that [Mexico does] what they promised to do, that there's an actual result, that we see a vast reduction in those [migration] numbers."

He said the arriving migrants from Guatemala, El Salvador and Honduras largely amounted to 'an economic migration that we need to stop with enforcement. We need to be able to repatriate people successfully."

McAleenan said that 'people can disagree with the tactics" - Trump's threat to impose a 5% tariff on Mexican imports starting Monday but that "Mexico came to the table with real proposals. We have an agreement that, if they implement, will be effective."

But he said Congress still needs to enact other immigration reforms, including the right to detain migrant families beyond 20 days and change the provisions of asylum requests to more closely align with the likelihood of whether migrants ultimately will be successful in their bids to stay permanently in the United States.