WASHINGTON/NEW YORK (Reuters) - The Trump administration on Tuesday said arrests at the U.S.-Mexico border fell in September for the fourth month in the row, and credited cooperation from Mexico and Central American countries in cracking down on migrants.

But due to record high numbers of crossings this Spring, nearly 1 million migrants were arrested and deemed inadmissible for entry to the United States in the 2019 fiscal year, which began in October 2017 and ends in September, Mark Morgan acting commissioner of the U.S. Customs and Border Protection (CBP), said at a press conference.

Morgan said there were just over 52,000 migrants either apprehended or encountered at the southwest border in September, down almost 65% from a peak in May of 144,000. That monthly total is still the highest September level since at least 2014, according to CBP statistics.

The bulk of arriving migrants are from Central America, many of them families, fleeing situations of violence and poverty at home and often seeking asylum in the United States.

Immigration has been a central issue for President Donald Trump who is gearing up to push it in his 2020 re-election campaign. His administration has taken a series of escalating measures to curb access to asylum and limit legal immigration. Many have drawn legal challenges.

In September, the U.S. Supreme Court allowed a controversial new rule to take effect, one that could curtail most asylum applications at the border while the court battle over its merits continues.

Morgan said the rule will be implemented this week. It requires most immigrants to seek safe haven in a third country they traveled through before they can apply for asylum in the United States.

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Another policy, the Migrant Protection Protocols, has been allowed to move forward temporarily by the 9th U.S. Circuit Court of Appeals. It has pushed more than 51,000 migrants back to Mexico since January to await their U.S. asylum hearings.

The Trump administration has pushed Guatemala, Honduras and El Salvador to sign deals to accept more asylum seekers closer to home. Few details have been released about how they will be implemented.

Many migrants are fleeing transnational gangs that operate in the entire region. Local asylum offices in Central America have few resources to handle a surge of new cases from neighboring countries.

Mexico has so far resisted signing any kind of “safe third country” agreement that would force migrants to seek refuge there instead of the United States. Morgan said he did not disagree with the foreign minister’s stance against such a deal.

“The cooperation that we are receiving with Mexico right now it is exactly where we need it to be,” Morgan said.