MEXICO CITY - US military officials met with a Mexican delegation in the southern Mexican border city of Tapachula on Tuesday to discuss security initiatives, sources familiar with the matter said on Wednesday.Lori Robinson, chief of the US Northern Command, and Kurt Tidd, head of the Southern US Command, attended the session, 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 US and Mexican officials since US President Donald Trump took office earlier in January. Trump has threatened 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 US Ambassador to Mexico Roberta Jacobson also attended.It was not clear who represented the Mexican government, but Mexico's Foreign Ministry said that Foreign Minister Luis Videgaray was not present.US Northern Command spokesman Michael Kucharek confirmed that Robinson and Tidd visited Mexico on Jan. 31 as part of "continued coordination in partner nation security," adding that the two men also went to Honduras and Guatemala.A spokesman for the US 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.The relationship between the United States and Mexico has become strained as 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.A summit between Trump and Mexico's President Enrique 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 to pay for the wall, further angering Mexico.