United Kingdom, Feb. 27 -- Alyson Hannigan says Michelle Trachtenberg "brought a loving energy" to 'Buffy The Vampire Slayer'.

The 50-year-old actress - who played Willow Rosenberg on the iconic teen drama - has paid tribute to her late co-star, who died this week aged 39.

Alyson wrote on Instagram: "I am deeply saddened by the news of Michelle's passing.

"She brought a loving energy to the set of Buffy. My thoughts are with Michelle's family and friends."

Trachtenberg joined the series for its fifth season as Sarah Michelle Gellar's titular character's sister Dawn Summers, and she remained on the show until its conclusion with season seven in 2003.

David Boreanaz, who played vampire Angel, wrote on his Instagram Story: "So very sad ...