Nina Nastasia, Lyric, Hammersmith, London

The best of the alternative country music celebrated in the Way Beyond Nashville festival (now in its third year at London's Barbican) tends to be the dark stuff: intense balladry about love and death. Adoptive New Yorker Nina Nastasia's music undeniably falls into that category - it has been described as "folk noir" - yet is far warmer and more personal.

Much of the warmth comes from Nastasia's voice. An odd-looking 37-year-old with black plaits rolled tightly on top of her head, she produces an incongruously clear, calm sound that that can also go incredibly high without a hint of shrillness. She is a singer you have to see live to realise just how beautiful her voice is, to hear that it is not a studio trick. The rest of the sound is made up of anxious brush-drumming, cello, violin, and insistent folk guitar plucking. This is a reduced version of the band that recorded Nastasia's three albums, Dogs, The Blackened Air and Run to Ruin, but it still conveys that deep sense of foreboding, fear even, that dark country does so well. It's as though the music is saying "there are bad things out there", but the voice is telling you "it's going to be alright".

The lyrics, though, are saying nothing of the sort. Instead they conjure up strange half-realised mysteries and dramas. With great economy of words and tone (she never sounds remotely self-pitying) Nastasia consistently manages to convey what something feels like without saying what it is. The impression is often that these songs were written for someone who she knows very, very well.

And then there are the heart-stopping moments - maybe three tonight - when her voice suddenly soars and sears into something quite different, a cry that stays bang on the note in a way that Courtney Love can only dream about. On "You Her and Me", Run to Ruin's most instantly captivating track, it happens on the line "white summer dresses". The torment the wailing suggests only becomes clear through what happens next in this strange story of a trip to the seaside.

The comparison with Courtney Love is apposite: although she plays acoustic folk, Nastasia's music has something in common with grunge rock, and her albums are produced by Steve Albini, who also helmed Nirvana's In Utero. Although Nastasia performs some great songs alone with her guitar, the ones that define her particular sound are those that build their drama around the quiet-loud-quiet dynamics of grunge, closing each song with a wild instrumental break. Except that where Nirvana would just bring a song crashing down in a wail of feedback, here those noises are being played right to the end on acoustic instruments. It comes across as the truly great "unplugged" concert no rock band has ever managed. Nina Nastasia is simply mesmerising.