U.S. prosecutors charged nearly 5,000 fewer drug cases in 2015 than three years earlier, after the Obama administration decided to target only the most serious cases, the Justice Department said on Monday.

A 14 percent decrease from fiscal 2012 to 2014 was followed by an additional 6 percent decline to 19,792 federal cases prosecuted in fiscal 2015, the department reported.

While state prosecutors can take on drug cases, federal prosecutors have been instructed since 2013 to charge fewer drug offenders to reduce disparities in sentencing that disproportionately affect minorities.

RELATED: Two Brooklyn men arrested in $10M illegal marijuana operation

The policy, known as Smart on Crime, was also intended to avoid charging certain non-violent, low-level drug offenders with the mandatory minimum sentence.

The Justice Department said 47 percent of the drug cases prosecuted in 2015 carried the mandatory minimum sentence, compared with 61.5 percent in 2012.

Some assistant U.S. attorneys had raised concerns that lower sentences would make defendants less willing to cooperate.

But Deputy Attorney General Sally Yates said the rate of cooperation has not changed. "It is always going to be in a defendant's interest to do whatever he or she can to be able to cooperate and to get a lower sentence," she told reporters at a news briefing.

Yates said she was hopeful bipartisan support for sentencing reform in the U.S. Congress would result in legislation this year, preventing policies like Smart on Crime from being dropped by the next administration.