Trump appeared to be referring to an April 13 incident in which Mexican troops reportedly questioned and pointed their weapons at two US troops conducting surveillance on the border.

"Mexico's Soldiers recently pulled guns on our National Guard Soldiers, probably as a diversionary tactic for drug smugglers on the Border. Better not happen again!" he tweeted.

"We are now sending ARMED SOLDIERS to the Border. Mexico is not doing nearly enough in apprehending & returning!"

US Northern Command said the two US soldiers were in an unmarked vehicle conducting "border support operations" on the American side of the border when they were approached by five to six Mexican military personnel

The incident occurred north of the Rio Grande in Texas, but in an unmarked area south of the border fence.

An inquiry "revealed that the Mexican military members believed that the US Army soldiers were south of the border. However, the US soldiers were appropriately in US territory," NORTHCOM said.

Defense officials told CNN the Mexican soldiers pointed their weapons at the US troops, removed a soldier's sidearm and returned it to their unmarked vehicle.

The command's statement said only that the Mexican soldiers departed the area "after a brief discussion between the soldiers from the two nations."

"Throughout the incident, the US soldiers followed all established procedures and protocols," it added.