He commented amid growing pressure to speak out after allegations that a top White House aide had abused two former wives.
He says, "Everyone knows that, and it nearly wouldn't even have to be said".
Kelly expressed his support of Porter, even after the initial allegations surfaced. An alleged abuser was allowed to remain in a position of power in the White House because an alleged abuser is living in the White House. He has denied the allegations.
There have also been mounting questions about when the White House knew about the allegations against Porter and why it didn't act sooner to remove him.
On Tuesday, she said the FBI's report had gone to the White House "personnel security office", which she said was staffed by civil servants rather than political appointees.
When reporters asked Trump about his former aide, the president wished Porter well, praised his performance in the West Wing and repeated Porter's claim of innocence. "There's a breakdown in the vetting system, and that needs to be addressed". Kelly has yet to give in his letter of resignation but said he would do so if Trump said so.
The FBI revelations were an embarrassment for the White House, where Sanders was forced to revise the official account of when top officials learned of the allegations against Porter.
"How do you have any job if you have credible allegations of domestic abuse?" Nothing had been decided or announced as of Wednesday morning, one senior White House official said.
The president's longtime personal attorney, Michael Cohen, now says that he paid $130,000 to a porn film star who allegedly had an extramarital affair with Mr. Trump in 2006. That included Don McGahn, the White House counsel, and Sarah Sanders, the press secretary. "Foreign intelligence services look for vulnerabilities in individuals with access to our most sensitive information".
The White House has declined to publicly detail the security clearance process or say which officials are operating on interim or full clearances.
Willoughby obtained a protective order against Porter in June 2010 after she said he violated their separation agreement and refused to leave her apartment, according to court records obtained by The Daily Mail.
Another White House staffer, National Economic Council policy aide George David Banks, told Reuters he had quit after he was told on Tuesday that he had not cleared a security check due to past marijuana use.
"It's unusual that so many have taken so long", he said. The official wasn't authorized to discuss the matter and spoke on the condition of anonymity.
The White House claims that the backlog of interim security clearances is a procedural effect of the review process carried out by the Federal Bureau of Investigation and White House Office of Security, which can take time to complete.
"The world witnessed firsthand the bravery of 156 women - so many of whom have represented our country at the highest levels of athletics in the Olympics - confronting a man they were told they could trust, as Larry Nasser was sentenced to prison for sexually assaulting them", House Majority Leader Kevin McCarthy, R-California, said in a statement. He called for limits on the information made accessible to those with temporary clearances - a practice that is now not followed in the West Wing, an official said.