MEXICO CITY (Reuters) - The United States has offered to help fund Mexico’s efforts to eradicate opium poppies, the U.S. assistant secretary for International Narcotics and Law Enforcement Affairs (INL) said on Friday, as Mexican heroin output increased again last year.

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“We would be prepared to support (opium eradication efforts) should we reach a basic agreement in terms of how they would do more and better eradication in the future,” William Brownfield of INL, part of the State Department, said in an interview.

“That is on the table, but I don’t want you to conclude that it’s a done deal, because we still have to work through the details,” he said, without specifying how much money the United States could provide.

The United States is in the midst of an opiates epidemic that has killed tens of thousands of people, and with much of its heroin coming from the mountains of Mexico, the issue has become a key topic of discussion between the Mexican government and the administration of U.S. President Donald Trump.

The U.S. offer to help fund Mexico’s war on poppy cultivation stands in stark contrast to Trump’s threats to rip up the North American Free Trade Agreement and force Mexico to pay for a wall along the U.S. border, and reveals the more subtle discussions taking place between the two governments.

Mexico’s president’s office, the Interior Ministry and the Foreign Ministry did not immediately respond to requests for comment.

Speaking on condition of anonymity because the figures are not yet public, a U.S. official said separately that the area of opium poppies under cultivation in Mexico reached 32,000 hectares in 2016, equivalent to about 81 tonnes.

In 2015, Mexico had 28,000 hectares under cultivation, almost triple the area in 2012, according to U.S. data.

Support for eradicating Mexico’s opium crop could come in various forms, Brownfield said. For example, the U.S. government could provide more vehicles, or pay for helicopter flights to access the isolated, mountainous regions where poppy is grown.

“If it’s a matter of having other sorts of equipment, we could talk about support in terms of equipment,” he said.

The INL will not write Mexico a blank check but is willing to help fund specific units involved in eradication, he said.

Mexico is engaged in fraught discussions with the Trump administration over drug trafficking, trade and immigration, and Trump focused on the heroin scourge in his election campaign.

Nonetheless, Brownfield said the two governments were making substantial progress.

“Our cooperation with the Mexican government on the heroin challenge is in fact good, and it is better than it has ever been in the past,” he said.

Brownfield also confirmed a Reuters report that Mexico’s army is allowing the United States and the United Nations to observe eradication efforts.