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 defense officials told CNN last week that two US soldiers were in an unmarked vehicle conducting surveillance on the American side of the border when they were approached by five to six Mexican military personnel.

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

Although the area was on the US side of the border north of the Rio Grande in Texas, it was south of the border fence.