"I am extremely concerned," King said on MSNBC's "Morning Joe." "What the President is telling us is not true. We can't vet the refugees ... There is no vetting as a practical matter. It is not a real vetting."
King said he was basing his view on classified briefings he has received as a member of Congress about the vetting process.
The Long Island Republican said "there are no databases" to check the information being provided by the refugees against, so if someone who is seeking entry into the U.S. says he has no terrorist involvement, American officials can't determine if that's true or not.
"We have no way of knowing," King said. "Unless we know who they are, we cannot allow them in. My job is to protect Americans, not to feel good about myself."
In the meantime, King said the U.S. should seek to find the Syrian refugees sanctuary in the Middle East.
He also said that he doesn't believe governors have the legal power to ban refugees from being settled in their stated, adding that it is a federal matter.
King wrote to President Barack Obama and Speaker Paul Ryan Tuesday calling for the immediate suspension of the resettlement of Syrian refugees in the U.S. "until an effective vetting and monitoring process that ensures our national security is in place."
Also Tuesday, Donald Trump tweeted "Is our president insane?" as the Republican presidential candidate warned about allowing Syrian refugees into the U.S.
Earlier, Rep. Seth Moulton, a Marine Corps veteran of the Iraq war, also appeared on "Morning Joe" and said he's looking for a plan from President Barack Obama.
"We need comprehensive plan to defeat ISIS," the Massachusetts Democrat said. "I am not confident we have that right now."
Moulton said Obama should address the nation and lay out a "long-term plan" for Iraq, Syria and Afghanistan.