WASHINGTON – The U.S. Border Patrol told its agents on Friday that when they confront suspected illegal immigrants crossing the frontier who throw rocks at them, they should try to take cover or move away rather than immediately open fire.

Michael J. Fischer, head of the Border Patrol, said in a conference call with reporters that immigrant smugglers were increasingly using rock-throwing as a way to drive away patrols.

Since 2010, agency personnel have opened fire 43 times, killing 10 people, in response to 1,713 rock-throwing attacks against them, Fischer said in the preface to a directive he issued to agents.

He said three officers had been killed in more than 6,000 assaults on agents since 2007.

Hundreds of thousands of people each year illegally cross the U.S. border with Mexico, which is also a major drug-running route into America.

In the directive, Fischer told agents not to open fire “unless the agent has a reasonable belief, based on the totality of the circumstances, to include the size and nature of the projectiles, that the subject of such force poses an imminent danger of death or serious injury.”

Border patrols have been criticized in recent years over concerns that agents may have sometimes been too quick to open fire. Sixteen members of Congress called for a review of use-of-force incidents and policy in 2012. The agency said in September it would train officers to defuse threats.

In one recent incident, an agent shot and killed a suspected illegal immigrant last month during a confrontation not far from the border with Mexico in the Otay Mountain area east of San Diego after the agent was pelted with rocks, the U.S. Border Patrol said.

The directive also said an agent should not fire on “a moving vehicle unless the agent has a reasonable belief, based on the totality of the circumstances, that deadly force is being used against an agent or another person present.”

It told supervisors in the Border Patrol, which is part of the U.S. Department of Homeland Security, to use alternatives to deadly force, including equipment to deflate tires and using “less-than-lethal equipment.”