By David Shortell, CNN

(CNN) -- Attorney General Jeff Sessions defended the administration's "zero-tolerance" immigration policy in a speech Monday as criticism over the resulting separation of parents and children entered a new week.

Speaking to a gathering of school resource officers in Reno, Nevada, Sessions blamed Obama-era border directives for forcing action on a trend of immigrants crossing as families that had "surged dramatically," and sought to associate the young children with high caretaking costs and public safety threats.

But he also tempered his rhetoric with a nod to the humanitarian backlash being driven by daily reports of the more than 2,000 boys and girls who had been separated from their parents and placed in detention centers and shelters across the country.

"We're going to continue to prosecute those adults who enter here illegally. We're going to do everything in our power, however, to avoid separating families. All federal agencies are working hard to accomplish this goal," Sessions said.

The Justice Department on Thursday asked a California court to modify a federal order limiting the ability of US officials to detain immigrant children for more than 20 days -- a central thrust of the executive order signed by President Donald Trump last week that was widely seen as a reversal of the hardline tactic.

Immigration experts have cautioned that the move is unlikely to succeed in the courts, and Sessions expressed frustration with his options on Monday.

"Let me tell you. This is a difficult and frustrating situation. I have spent hours, days, months -- so has our legal -- working with Homeland Security, working with Border Patrol, our ICE officers, who are trying to deal with the loopholes and problems that are created in our system, and lawyers over the years have developed more and more clever ways to frustrate the normal expected enforcement of these laws," Sessions said.

"And while we want to keep these families together, we need Congress to act. We really do," he said.

Sessions also used his time in front of the school resource officers conference to list examples of violence by MS-13 in the US in grisly detail and, at one point, tied the gang directly to the population of immigrant children.

"MS-13 is recruiting children who were sent here as unaccompanied minors. They come here with not roots and minimal connections and they seek to recruit from them. And some come here, and are brought here, and sent here by the gangs in Central America to strengthen and replenish the MS-13 gangs," he said.