US Secret Service Director Kimberly Cheatle admitted to lawmakers the attempted assassination of Donald Trump was "the most significant operational failure at the Secret Service in decades."

She was testifying at a US House of Representatives hearing into her agency's failure to prevent a would-be assassin from wounding a Republican presidential candidate.

"We failed," Cheatle, facing calls to resign, said during testimony before the House Committee on Oversight and Accountability.

"As the director of the United States Secret Service, I take full responsibility for any security lapse," Cheatle said. 

The Secret Service director also said she apologized to Trump in a phone call after the assassination attempt.

Trump's security was beefed up

Republican claims that the Secret Service was denied resources to protect Trump were dismissed by Cheatle, who said the former president's security had been increased before the shooting.

"The level of security provided for the former president increased well before the campaign and has been steadily increasing as threats evolve," Cheatle said.

She aslo added that the Secret Service provided the security the Trump campaign requested for the rally. 

"What I can tell you is that for the event on July 13, the details that were requested, the assets that were requested for that day, were given," Cheatle said.

Cheatle faces storm of bipartisan criticism

Lawmakers repeatedly asked Cheatle why she had not yet resigned from her job yet.

"The Secret Service has thousands of employees and a significant budget. But it has now become the face of incompetence," the Republican chairman of the House Oversight Committee, Representative James Comer of Kentucky said. 

"If you have an assassination attempt on a president, former president or a candidate, you need to resign," Democratic Representative Ro Khanna said. "You cannot go leading the Secret Service agency, when there is an assassination attempt on a presidential candidate."

Cheatle however insisted she was the "right person" to lead the agency.

Monday's hearing was the first round of congressional oversight regarding the assassination attempt. FBI Director Christopher Wray is scheduled to appear before the House Judiciary Committee on Wednesday.

What do we know about the attempted assassination?

Trump was wounded in the ear in the July 13 Pennsylvania rally shooting that left one spectator dead and two attendees seriously injured. 

The 20-year-old gunman, Thomas Matthew Crooks, opened fire with an AR-style rifle just minutes into Trump's speech. Twenty-six seconds after firing the first of eight shots, Crooks was shot and killed by a Secret Service sniper.

Investigators have determined that Crooks, who lived in a town about 80 kilometers (50 miles) from Butler, was a lone gunman and had no strong ideological or political leanings.

dh/lo (AP, AFP, Reuters)