The U.S. decision to send anti-personnel landmines to Ukraine — a major policy shift slammed by rights groups — was triggered by a change in Russian battlefield tactics favoring infantry over mechanized units, U.S. Defense Secretary Lloyd Austin said Wednesday.

"They don't lead with their mechanized forces anymore," Austin told reporters while on a visit to Laos. "They lead with dismounted forces who are able to close and do things to kind of pave the way for mechanized forces."

The Ukrainians "have a need for things that can help slow down that effort on the part of the Russians."