Mexican Foreign Minister Marcelo Ebrard said the country was committed to continuing to work to keep migrants from Central America from reaching the U.S. border. Pushing back against Trump’s charge that Mexico was doing “nothing” to help, the government said 250,000 more immigrants would reach the United States in 2019 without its efforts.

Ebrard said, however, that a proposal favored by some U.S. officials to designate Mexico a “safe third country,” which would force Central Americans seeking asylum in the United States to apply for it instead in Mexico, was not an option.

“An agreement about a safe third country would not be acceptable for Mexico,” Ebrard told reporters in Washington. “They have not yet proposed it to me. But it would not be acceptable and they know it.”

Such a designation would in theory cut the number of new Central American asylum seekers to almost zero, and removing it from the discussion leaves fewer negotiating options available for avoiding the 5% tariffs due to kick in next week.