A free-thinking group of Roman Catholic priests has promised to continue blessing same-sex couples - in defiance of the Vatican.

The Vatican said this week that priests can't bless same-sex unions because God "can't bless sin," in a decision that disappointed gay Catholics, Reuters reported.

"It is not licit to impart a blessing on relationships or partnerships...that involve sexual activity outside of marriage...as is the case of the unions between persons of the same sex," the Vatican statement said. 

In response, a number of Catholic priests have opposed the church's decree, including an Austrian-based group known as the Parish Priests Initiative.

Parish Priests Initiative said it was appalled by the church's decree. "This is a relapse into times that we had hoped to have overcome with Pope Francis," Reuters quoted the group as saying.

"We will - in solidarity with so many - not reject any loving couple in the future who ask to celebrate God's blessing, which they experience every day, also in a worship service," the group said.

The initiative was founded in 2006 with nine members and now has about 350 members from priests and deacons and more than 3,000 laypeople led by church reform leader Fr. Helmut Schüller.

The group has often disagreed with the Vatican. It pushed the church to allow priests to marry and allow women to become priests. It has said it will disobey the church mandate by giving communion to Protestants and divorced Catholics who remarry.

In 2012, Pope Benedict XVI was angered by the group's disobedience and stripped Schüller of the title of "monsignor" and saying he was no longer a "Chaplain of His Holiness."

In some countries ministers and parishes have begun blessing same-sex unions in lieu of marriage. Conservatives in the 1.3 billion-member church have become alarmed at this.