The Filipino champion's camp announced on Wednesday that he would challenge undefeated Horn in Brisbane in July.
The Philippine world boxing champion's camp announced in Manila on Wednesday that Pacquiao would challenge undefeated Horn in Brisbane in July.
But the consortium backing the fight, including the Queensland state government, and promoters Duco Events and Top Rank Boxing, were not ready to make a formal announcement.
"Until (the) process is completed, where all parties are contractually bound, there is no event to be announced," Duco said in a statement.
"Media speculation has seen a lot of expectations created, and then dashed in months gone by with this event."
The Filipino's advisor told AFP in Manila on Wednesday the world champion, 38, would face Horn, 29, in on July 2.
It is the second time the bout has been touted, after Pacquiao earlier this year instead decided to pursue a big money fight with British boxer Amir Khan in the United Arab Emirates until the deal fell through.
Despite Duco's caution, Pacquiao's camp said Thursday it was keen for a quick win over Horn to set up a higher-profile title fight with Khan.
"We're not able to fight Amir Khan because of Ramadan, so we decided to take this fight in the interim," Pacquiao's advisor Michael Koncz told Fox Sports in Australia.
Koncz said Pacquiao's fans, when polled by the eight-division world champion on Twitter, preferred him to fight Khan, with that prospect now on the table for October or November.
"We signed it, so yeah, we're excited about it. But again, my preference was to fight Amir Khan," Koncz said. "That didn't work out. We had this on the table so we decided to take it.
"Manny will train and do the best he can and hopefully we get the fight over with very quickly, we get out of there, we say hi to the Australian fans and media and come home," he added.
"Frankly and honestly, and nothing against Jeff Horn -- I've never met the kid, I don't know him personally -- but the name recognition is I guess why the fans picked Amir Khan."
Koncz said Pacquiao would be staking his World Boxing Organization welterweight crown against the Australian.
The Filipino, a national hero in his home country, briefly retired early last year and won a seat in the Philippine senate.
But the retirement was short-lived as Pacquiao made a successful comeback against Jessie Vargas in Las Vegas in November.