Speaking ahead of the meeting at the Vatican on Saturday, Dr Tveit said he expected to discuss some of the challenges facing the global body of Christ, including ongoing efforts towards church unity and the plight of Christians in the Middle East.
"It is important that we speak honestly in this meeting about the challenges we have," Dr Tveit said.
"There are expectations for the ecumenical movement that have not been fulfilled, and there are tensions arising in and between churches.
"Therefore, it is even more important now to stay with our commitment and to reflect what this call implies in our daily life as Christians all over the world."
Dr Tveit acknowledged that there were still obstacles to visible unity, such as differences over the eucharist and ordained ministry.
However he said that the WCC was committed to seeking visible unity around the world and with the Catholic Church.
He said hoped the WCC and Vatican could focus on their common calling to mission and unity and "what it is possible for us to do together".
"What encourages us is that this calling is something that many share as a high priority, and I know that this is the case with Pope Benedict," he said.
"I very much appreciate what Pope Benedict has said on many occasions, how he is committed to the work of unity, how he is committed to the mission of the church, to work for justice and peace and to the sharing of the church with new generations."
Dr Tveit will appeal to Pope Benedict to continue to engage in common action and advocacy to support Christian communities of the Middle East.
He said: "Our witness to the gospel, our support for justice and peace, solidarity with the oppressed, initiatives for Jewish-Christian and Muslim-Christian dialogue and cooperation, all of this comes together in the Middle East, and especially in Jerusalem."