Abba star Bjorn Ulvaeus says two new songs - one a danceable pop tune and the other 'Nordic sad but happy at the same time' - from the Swedish foursome are on the way.

Ulvaeus revealed the details of the two new songs on the red carpet at the world premiere of Mamma Mia! Here We Go Again in London on Monday.

The band had already announced they were getting back together to make new material ahead of their digital avatar tour.

"One of them is a pop tune, very danceable," Ulvaeus said of the new material.

"The other is more timeless, more reflective, that is all I will say. It is Nordic sad, but happy at the same time."

He said the singing of Agnetha Faltskog and Anni-Frid Lyngstad lifted the sad song.

"I think that exuberant quality of the two ladies (Agnetha Faltskog and Anni-Frid Lyngstad) together singing, that makes the saddest song a bit uplifting, anyway I think that is what happens and that is very organic, that isn't calculated, that just happened that way."

Bandmate Benny Andersson hinted he had a third song on his mind.

"Yeah it is but don't tell anyone. If we do a third one, it won't be out until we go on the road," he said, adding a third album was unlikely.

"Doing another album is like doing Mamma Mia 3, what is the point unless there is a good reason, good script, good environment? It is such a joyful experience, so that is a reason to do it."

Andersson also said he was determined that any sequel to Mamma Mia would have value that was more than just financial, so he wanted to wait until there was a good script.