A US senator has made a powerful speech blaming politicians for the country's latest school massacre.

Nikolas Cruz, 19, allegedly stormed his former high school in south Florida on Wednesday and opened fire on students and teachers, killing 17 people and injuring at least 14 others.

Reacting to the mass shooting on Wednesday, Democratic senator Chris Murphy said "it's time for Congress to get off its ass and do something" about gun violence in the US.

"This happens nowhere else other than the United States of America," Senator Murphy said.

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"This epidemic of mass slaughter, this scourge of school shooting after school shooting, it only happens here not because of coincidence, not because of bad luck, but as a consequence of our inaction.

"We are responsible for a level of mass atrocity that happens in this country with zero parallel anywhere else.

"As a parent, it scares me to death that this body doesn't take seriously the safety of my children and it seems like a lot of parents in south Florida are going to be asking that same question later today."

Senator Murphy is a representative for Connecticut, which was the scene of Sandy Hook Elementary School shooting, which saw 20-year-old Adam Lanza shoot and kill 20 young students and six adult staff in 2012. He has been a strong advocate for gun control since that atrocity, but the US congress has yet to enact any laws that have curbed school gun violence.

Wednesday's incident at Marjory Stoneman Douglas High School was the 18th shooting at a US school this year. And it's not even March.

Research by gun control advocacy group Everytown for Gun Safety shows that there have been nearly 300 school shootings in the US since 2013, an average of about one per week.