Celtic boss Ronny Deila admits that his side must improve despite cruising into the Champions League third qualifying round with a 6-1 aggregate victory over Stjarnan.

Celtic manager Ronny Deila has insisted that his side must improve despite their 6-1 aggregate victory over Stjarnan in a Champions League qualifier.

The Bhoys were given an early scare in Iceland when they fell behind, but two late goals added gloss to a 4-1 victory on the night, adding to their 2-0 triumph in the first leg.

Deila was not entirely happy at the final whistle, however, criticising his team's start as well as the state of the artificial pitch.

"It's difficult to say what the performance was but when you play on artificial pitch that's dry it's sticky and everything goes slow," he told reporters.

"We were sloppy and a bit stressed but we got the goal and more tempo. In the second half there was only one team on the pitch. There's no problem to play artificial but when you don't water a pitch it's totally different. We always ask for a pitch to be watered but they said it was broken.

"We didn't want to concede an early goal but it happens sometimes in football and you need to bounce back. 4-1 is a positive result and I'm looking forward to the next match."

Deila's side will now face FK Qarabag of Azerbaijan in the next round, and he insists that a repeat performance will not be good enough.

"We need to improve. Qarabag are a good team and I watched their last game against Rudar. It's a big step up," he added.

Nir Biton, Charlie Mulgrew, Leigh Griffiths and Stefan Johansen got the goals for Celtic in Iceland.