MEXICO CITY (Reuters) - A Mexican delegation met U.S. military officials in southern Mexico on Tuesday to discuss security initiatives, sources said on Wednesday, as the two countries try to find common ground in tough negotiations over trade, security and immigration.

FILE PHOTO - U.S. Air Force General Lori Robinson, Pacific Air Forces commander, addresses airmen at Andersen Air Force Base, Guam July 10, 2015. REUTERS/U.S. Air Force/Senior Airman Katrina M. Brisbin/Handout via Reuters

The relationship between the United States and Mexico has become strained after U.S. President Donald Trump vowed to build a wall between the two countries to keep out illegal immigrants, drug dealers and criminals and make Mexico pay for it.

Lori Robinson, chief of the U.S. Northern Command, and Kurt Tidd, head of the Southern U.S. Command, attended the talks, two people familiar with the matter said. The sources spoke on condition of anonymity so they could discuss the meeting.

It was one of the first known meetings between U.S. and Mexican officials since Trump became president in January, after threatening to upend years of cooperation between the two neighbors with divisive rhetoric on trade and security.

One of the sources said the talks focused on Mexico’s commitment to securing its southern border to keep out criminals and illegal immigrants. The source said that U.S. Ambassador to Mexico Roberta Jacobson also attended.

Mexico’s Foreign Ministry said Foreign Minister Luis Videgaray was not present. A third source said Socorro Flores, deputy minister for Latin America and the Caribbean, represented Mexico at the meeting.

U.S. Northern Command spokesman Michael Kucharek confirmed Robinson and Tidd visited Mexico on Tuesday as part of “continued coordination in partner nation security,” adding that the two also went to Honduras and Guatemala.

A spokesman for the U.S. embassy in Mexico City said: “The southern border visit has been planned for months and gave U.S. authorities a chance to learn first-hand about the challenges Mexico faces in protecting its borders.”

The spokesman did not provide further details.

Mexico denied media reports on Wednesday suggesting that Trump, on a recent call with Mexican President Enrique Pena Nieto, had threatened to send U.S. troops into Mexico if the country did not step up efforts against its drug cartels.

CNN later published what it said was the real transcript from the call, showing Trump taking a more measured stance, and offering U.S. help to combat Mexico’s “tough hombres.”

Mexico’s presidential spokesman, Eduardo Sanchez, was not immediately able to comment on CNN’s report but said “it sounded more logical.”

A summit between Trump and Pena Nieto that had been planned for January was canceled after Trump tweeted that it would be better for the Mexican leader not to come to the summit if Mexico would not pay for the wall. Pena Nieto canceled.

Trump has threatened to levy a hefty tax on imports from Mexico to the United States.

Mexico said on Wednesday it expected to begin formal talks on renegotiating the North American Free Trade Agreement from around the beginning of May.