Attorney General Jeff Sessions said Friday that the Justice Department is ramping up steps to investigate and prosecute government leakers, a day after transcripts of President Trump's calls with two foreign leaders were leaked to the Washington Post.

"Criminals who would illegally use their access to our most sensitive information to endanger our national security are in fact being investigated, and will be prosecuted," he said in a press conference.

Sessions stressed that his department doesn't confirm or deny specific investigations, but made it clear there are many more leak investigations happening today than there were less than a year ago.

Since January, Sessions said the department "has more than tripled the number of active leak investigations" compared to the number pending at the end of the Obama administration.

The Justice Department has also received nearly as many criminal referrals "involving disclosures of classified information as we've received in the previous three years combined," Sessions said, revealing that the department has already charged four people with unlawfully disclosing classified information.

Only one of those charges has been announced by the Justice Department: 25-year-old government contractor Reality Leigh Winner, a former NSA contractor who is accused of leaking information regarding Russia's attempts to hack voting systems.

Sessions also stressed repeatedly that a way to solve the problem is to change the culture in government, which routinely leaks information.

"Prevention is what is required. An investigation of a leak is too late, really. The damage is done," he said. "This culture of leaking must stop.

"I have this warning for would-be leakers: Don't do it," Sessions added.

The Justice Department is also reviewing its policy on issuing subpoenas to reporters, Sessions revealed.

Sessions was joined by Director of National Intelligence Dan Coats, Deputy Attorney General Rod Rosenstein and National Counterintelligence and Security Center Director William Evanina from the Justice Department headquarters on Friday in Washington.

Sessions said he had directed Rosenstein and FBI Director Christopher Wray to be more involved with investigations into leaks by actively monitoring every single one.

Sessions also directed the National Security Division and U.S. attorneys to prioritize cases involving the criminal leaks of classified information, and said the FBI would be creating a new counterintelligence unit to oversee that task.

Trump, who has criticized Sessions in recent weeks, said he wanted Sessions to crack down on the leaks that have plagued his administration since he took office.

On Twitter, Trump said his attorney general had taken a "VERY weak position" on "Intel leakers."

Sessions told Fox News' Tucker Carlson last week that he has "not been happy with the past prosecutions and investigations of criminal leaks."

The Washington Post released the transcripts of Trump's calls in late January with Mexican President Enrique Peña Nieto and Australian Prime Minister Malcolm Turnbull and noted the transcripts had notes indicating the National Security Council had classified them. The full transcripts, the Post said, were "produced by White House staff" and based on records kept by White House notetakers.

"I strongly agree with the president and condemn in the strongest terms the staggering number of leaks undermining the ability of our government to protect this country," he said of the Post report.

"No one is entitled to surreptitiously fight to advance battles in the media by revealing sensitive government information," Sessions added. "No government can be effective when its leaders cannot discuss sensitive matters in confidence, or talk freely in confidence with foreign leaders."