"... in a series of meetings and telephone conversations, senior Iranian officials assured the Secretary-General that Iran understood and supported the basis and goal of the Conference, including the Geneva Communiqué. The Secretary-General is deeply disappointed by Iranian public statements today that are not at all consistent with that stated commitment. He continues to urge Iran to join the global consensus behind the Geneva Communiqué. Given that it has chosen to remain outside that basic understanding, he has decided that the one-day Montreux gathering will proceed without Iran’s participation."
The issue of whether to invite Iran had been a point of contention, with the UN and Russian government pressing the United States to agree to Iran's participation. As recently as January 13, U.S. Secretary of State John Kerry sparred with his Russian counterpart and a UN envoy during a press conference about Iran's potential attendance at Geneva II, with Kerry citing concerns about Iran's support of Hezbollah both within Syria and across the border in Lebanon as well as ambiguity whether Iran was a supporter of the Geneva I agreement.
The UN is the convener of Geneva II, with both the U.S. and Russia serving as co-hosts for the peace talks.