The sister of a student who hid inside a classroom during the school mass shooting in Florida has shared the chilling text messages they exchanged while the building was in lockdown.

At least 17 people were killed when former student Nikolas Cruz, who had been expelled last year for disciplinary reasons, allegedly opened fire at Marjory Stoneman Douglas High School in Parkland on 14 February.

A woman, who gave her name only as Kaitlin, posted screenshots on Twitter showing the terrifying text messages sent to her by her sister Hannah, a student at the school who was hiding in a classroom alongside someone who got shot. The other student's condition was unknown but Hannah was later confirmed to be safe.

Hannah said she could not get any phone signal to call her parents or 911, and begged her sister to make the calls. Kaitlin in return insisted that Hannah keep texting to show she was safe.

Hannah wrote: "Kaitlin there is a shooter on campus. I am not joking. Call 911 please. Send them to Douglas." Her shocked sister replied: "Hannah what. Are you joking rn [right now]."

Hannah then shared the horrific news that someone in her classroom had been shot. "Kaitlin I am not joking they just shot through the walls someone in my class is injured. I am not joking. Call mom and dad."

After Kaitlin promised to call 911 and her parents immediately, she said: "I love you so much."

Heartbreakingly, Hannah replied: "I'm so scared I can't make any calls I love you." Kaitlin continued to request for more information about the shootiing, and urged her sister to keep texting.

Hannah said: "I don't know it's silent. I don't know if the police are here." Her next texts become even more chilling. "I am so scared Kaitlin. Tell them I love them [her parents] so much." She repeated: "I'm so scared. Tell them I love them so so much."

Kaitlin kept reassuring Hannah she was going to be fine and then asked for more details about the injured student, but her sister was unsure. "I just know she got hit." Hannah also said she heard "a lot" of shots and that the police had arrived in the building.

Kaitlin's tweet has gone viral and been liked more than 17,000 times, with many declaring their support and sympathies and some saying the exchange had brought tears to their eyes. Kaitlin later revealed how she felt while texting Hannah: "It was really hard and my mind was racing everywhere but I tried to do my best from work to get her helped and I didn't let her stop texting me."

Fifteen people died at the school and two died after being taken to hospital. A further 15 injured were transported to hospital, according to Broward County Sheriff Scott Israel.

When police arrived at the scene, hundreds of students were fleeing the school - with suspect Cruz disguising himself among them. It is believed he set off a fire alarm to force students to leave their classrooms before opening fire.

Sherriff Israel tweeted: "It's catastrophic. There really are no words."

The attack is the deadliest school shooting since 26 people were killed at Sandy Hook Elementary School in Connecticut in 2012.

Florida school shooting evacuation
People are brought out of the Marjory Stoneman Douglas High School after a shooting at the school that reportedly killed and injured multiple people on February 14, 2018 in Parkland, Florida. Numerous law enforcement officials continue to investigate the crime scene Joe Raedle/Getty Images