WASHINGTON (Reuters) - The head of counter-narcotics at the U.S. State Department stressed on Thursday the importance of working with Mexico in countering the flow of killer drugs, a cooperative effort that has been called into question by U.S. President Donald Trump’s plan to build a border wall.

William Brownfield estimated that more than 90 percent of the heroin consumed in the United States currently comes from Mexico, along with most of the notorious killer drug fentanyl.

The White House has said a wall will stem the flow of drugs, crime and illegal immigration, but Mexico has warned that a breakdown in relations could affect cooperation on these very issues.

While drugs continue to stream across the border, Brownfield argued that the United States was better able to deal with the problem today than a decade or two ago because of international cooperation, including joint operations and intelligence sharing with Mexico.

Briefing reporters on the release of the State Department's annual International Narcotics Control Strategy Report, the career foreign service officer appointed by former President Barack Obama said counter-narcotics cooperation with Mexico was at "historically high levels," which boded well for the future. (Report: bit.ly/2mf2JZh)

“In a sense, we have developed a law-enforcement cooperative wall at this point, without actually having the physical construction of a wall,” said Brownfield, the assistant secretary for international narcotics and law enforcement.

He did not respond directly when asked if a wall would help stop the flow of drugs from Mexico, but said President Trump had made his intentions clear, and any new “opportunities or tools” made available would be integrated into efforts to stop cross-border trafficking.

“But I would also say ... that we will do it in cooperation with our counterparts in the government of Mexico,” Brownfield said.

Mexicans have been angered by Trump’s calls for U.S. companies not to invest in Mexico, his insults to immigrants, and threats to make Mexico finance a border wall.

Mexico Economy Minister Ildefonso Guajardo has been particularly outspoken, saying that if the U.S.-Mexico relationship sours, his country will have less reason to cooperate on drugs and immigration.

Brownfield said fentanyl was being trafficked by the same organizations moving heroin and other opioids from Mexico into the United States.

Brownfield said most of the fentanyl entering the United States originated in China. He said Washington and Beijing planned in coming weeks to jointly support a UN resolution to put international controls on several precursors needed to produce fentanyl.