Tributes have been paid to actress Michelle Trachtenberg, who has died unexpectedly at the age of 39.

The star was known for playing Dawn Summers, the younger sister of Sarah Michelle Gellar's titular character in Buffy, The Vampire Slayer, and starred as Georgina Sparks in the teen drama Gossip Girl.

Trachtenberg was discovered unresponsive at her Manhattan apartment on Monday morning by her mother, but she was pronounced dead at the scene.

Subsequently it was revealed that the actress had recently undergone a liver transplant, although the exact cause of death will remain unknown as her family refused for an autopsy to be held on religious grounds.

Gellar led tributes to her former co-star, quoting a poignant line of dialogue between their characters on the show: "Michelle, listen to me. Listen. I love you. I will always love you. The hardest thing in this world, is to live in it. I will be brave. I will live... for you."

James Marsters, aka Spike, wrote on Instagram: "My heart is heavy today. We have lost a beautiful soul. Michelle was fiercely intelligent, howlingly funny, and a very talented person. She died much too young, and leaves behind scores of people who knew and loved her."

Kristine Sutherland, who played Buffy and Dawn's mother Joyce, said in a statement: "I am devastated by the news of Michelle's death. The day she joined the Buffy family she brought a joy and innocence with her along with her talent. She was lovely to work with and will be missed. My heart goes out to her family."