US President Barack Obama on Friday vowed to take action to stop gun violence in response to online petitions signed by more than 400,000 people after last week's elementary school massacre.

"In the days since the heartbreaking tragedy in Newtown, Connecticut, hundreds of thousands of you, from all 50 states, signed petitions asking us to take serious steps to address the epidemic of gun violence in this country," Obama said in an online video. "We hear you."

Obama has called on Congress to pass legislation banning military-style assault rifles and high-capacity ammunition clips. It would would also close loopholes that allow people to purchase guns without background checks.

He has also appointed Vice President Joe Biden to head a task force to explore ways to prevent mass shootings, including by improving access to mental health care, and addressing depictions of violence in popular culture.

"I will do everything in my power as president to advance these efforts, because if there's even one thing we can do as a country to protect our children, we have a responsibility to try," Obama said in the video."But as I said earlier this week I can't do it alone. I need your help."

Obama called on ordinary citizens, law enforcement officials and gun owners to campaign publicly and petition Congress in support of his reforms.

Last Friday's massacre of 26 people, including 20 children, at Sandy Hook Elementary School -- the latest in a series of mass shootings over the past two years -- has galvanized support for reforms aimed at stemming gun violence.

More than 400,000 people have signed "We the People" petitions on the White House's website calling for action on gun violence, making it one of the most popular issues since the launch of the site, a White House official said. One such petition set the record for being the fastest ever to reach 25,000 signatures, the official said.