The White House has unveiled gun control measures that require more gun sellers to get licences and more gun buyers to undergo background checks.
They are moves US President Barack Obama said were well within his authority to implement without congressional approval.
The US Bureau of Alcohol, Tobacco, Firearms and Explosives now will require that people who sell guns at stores, at gun shows or over the internet be licenced and conduct checks, officials said.
The ATF was finalising a rule requiring background checks for buyers of dangerous weapons from a trust, corporation or other legal entity as well.
Mr Obama, speaking to reporters before the measures were made public, said they were consistent with the US Constitution's Second Amendment, which protects the right to bear arms.
"These are not only recommendations that are well within my legal authority and the executive branch, but they're also ones that the overwhelming majority of the American people, including
gun owners, support," Mr Obama said during a meeting with US Attorney-General Loretta Lynch and other advisers.
Mr Obama is igniting a political firestorm by bypassing congress with the measures, which could spark legal challenges. Republicans say Mr Obama is misusing his powers.
Shares in gun makers Smith & Wesson Holding Corp and Sturm Ruger & Co Inc rose against a falling stock market on Monday (local time) in anticipation of increased gun sales, as has happened before when the White House considered weapon sales reform.
Stymied by congress' inaction on gun control, the president asked his advisers in recent months to examine new ways he could use his executive authority to tighten gun rules unilaterally after multiple mass shootings generated outrage nationwide.
The White House had drafted a proposal on licences previously but was concerned it could be challenged in court and hard to enforce.
The president's use of executive action launches his final year with a move that Republicans say exemplifies misuse of his powers. Congress, which is controlled by Republicans, rejected Mr Obama's proposals for legislation to tighten gun rules in 2013.
"The president is at minimum subverting the legislative branch, and potentially overturning its will," Republican Speaker of the House of Representatives Paul Ryan said in a statement before the White House announcement.
Republicans have called for more focus on mental healthcare rather than measures to restrict gun ownership. The White House said it would ask congress for US$500 million in its 2017 Budget to boost access to mental healthcare.