Former US President Barack Obama has warned of a "political crisis of the sort that we haven't seen before" in the wake of the killing of Charlie Kirk.

At an event in Pennsylvania on Tuesday, Obama said he did not know Kirk and disagreed with many of his views, but called the killing "horrific and a tragedy".

He criticised Donald Trump's remarks towards his political opponents and pointed to previous Republican presidents who, he said, emphasised national unity in moments of high tension, US media report.

In response, the White House called Obama the "architect of modern political division".

Kirk, 31, died of a single gunshot wound while speaking at Utah Valley University in Orem on 10 September.

On Tuesday, Tyler Robinson, 22, was formally charged with Kirk's murder, weapons offences and other charges. Prosecutors said they would seek the death penalty.

Utah County Attorney Jeffrey Gray said Robinson had sent text messages which allegedly said he shot Kirk because he "had enough of his hatred".

Before Robinson was captured, top Trump allies pinned blame for the killing on left-wing activists and rhetoric from Democratic lawmakers and their supporters.

Attorney General Pam Bondi has suggested that the administration will crack down on "hate speech" – although there is no specific US hate speech law. Vice-President JD Vance has led calls to expose people who celebrated or condoned Kirk's killing or were critical of him after his slaying.

"Call them out, and hell, call their employer," Vance said as he guest-hosted Kirk's podcast.

Speaking in Erie, Pennsylvania, Obama said: "I think at moments like this, when tensions are high, then part of the job of the president is to pull people together."

He urged Americans to "respect other people's right to say things that we profoundly disagree with".

Obama praised Utah Governor Spencer Cox, a conservative Republican who he said had shown "that it is possible for us to disagree while abiding by a basic code of how we should engage in public debate".

He also endorsed the response of Pennsylvania Governor Josh Shapiro, a Democrat, whose official residence was firebombed earlier this year in what police called a targeted attack.

The former president contrasted those reactions with comments made by Trump and his allies.

Obama said that he did not use a 2015 mass shooting by a white supremacist at a black church in South Carolina to go after his political enemies, and pointed out that after the 11 September 2001 attacks, President George W Bush "explicitly went out of his way to say, 'We are not at war against Islam'."

"And so when I hear not just our current president, but his aides, who have a history of calling political opponents 'vermin', enemies who need to be 'targeted,' that speaks to a broader problem that we have right now and something that we're going to have to grapple with, all of us," Obama told the crowd, according to reports.

In a statement to the BBC, a White House spokesperson rejected the allegations and accused Obama of stoking division while he was president.

"Obama used every opportunity to sow division and pit Americans against each other," the spokesperson said.

"His division has inspired generations of Democrats to slander their opponents as 'deplorables,' or 'fascists,' or 'Nazis.'"

After leaving office, US presidents generally tend to temper criticism of their successors, however in recent months Obama has hit out at Trump's moves against universities and judges, and has also criticised Democratic party leaders for failing to push back harder against White House policies.