The rift between President Obama and FBI Director James B. Comey over the impact of the Black Lives Matter movement deepened Thursday, with the White House again disputing Mr. Comey’s assertion that it’s the cause of spiking crime in major cities.

“We just need to make sure that our policy approach to addressing this situation is rooted in evidence and facts,” said White House press secretary Josh Earnest. “It’s clear that we don’t have enough evidence at this point.”

For the second time in recent months, Mr. Comey said Wednesday that he believes a spike in violent crime in many cities may be due to officers’ fears of appearing on Internet videos confronting suspects. He told reporters that a “viral video effect” is leading to less aggressive policing that “could well be at the heart” of an increase in murders in many cities.

“There’s a perception that police are less likely to do the marginal additional policing that suppresses crime — the getting out of your car at two in the morning and saying to a group of guys, ‘Hey, what are you doing here?’” Mr. Comey said.

Although violent crime rates nationwide are near historic lows, a surge in more than 40 big cities is causing concern across government agencies trying to understand and respond. Crime rates have risen in those cities in the first three months of 2016, according to a briefing the FBI director received on Wednesday.

He said two cities that stood out especially in the latest tallies were Las Vegas and Chicago. In Chicago murders are up 54 percent and shootings up 70 percent over the same period last year.

“I don’t know what the answer is, but holy cow do we have a problem,” he said. “It’s a complicated, hard issue, but the stakes couldn’t be higher. A whole lot of people are dying.”

Last fall, Mr. Comey referred to the trend as “the Ferguson effect,” a reference to the Missouri city where a white officer shot unarmed black teen Michael Brown, sparking the Black Lives Matter movement, with which the president has expressed support.

Mr. Earnest said Mr. Obama last year directed the Justice Department to work with communities affected by the surge in violent crime, resulting in a program to capture more violent fugitives.

“That six-week initiative resulted in the arrest of more than 8,000 gang members, sex offenders and other violent criminals,” Mr. Earnest said. “That is an indication of the important role that federal law enforcement can play in supporting the work of local law enforcement in these communities.”

But he said Mr. Comey really doesn’t know the reasons for the surge in urban crime.

“The FBI director actually made clear that he didn’t know exactly what was going on either,” Mr. Earnest said. “The president’s point is we need to make policy decisions that are based on facts and evidence and not anecdotes. There is still no evidence to substantiate the claim that the increase in violent crime is related to an unwillingness of police officers to do their job.”

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