Jeff Horn has started fishing for a fight with IBF welterweight champion Errol Spence as his rematch with Manny Pacquiao starts to find itself on shaky ground.

Horn's call-out to Spence on social media on Monday was symptomatic of his desire to get the wheels moving on another bout before the end of the year as the wheels spin on any rematch with the Filipino legend.

Horn stunned most of the boxing world when he dethroned Pacquiao at Suncorp Stadium at the start of July, winning a unanimous decision and earning himself the WBO welterweight title. Spence won the IBF version in May.

Pacquiao had a rematch clause in his contract and could yet exercise it towards the end of the year, with November the most likely window given his duties in the as a senator in the Philippines.

Who's next?: Jeff Horn lands a left on Manny Pacquiao at Suncorp Stadium.... now he wants a new opponent. AAP

Questions about a venue – Melbourne had been tipped given the unsuitability of sub-tropical Brisbane towards summer – could now be pointless as Pacquiao continues to stall on any word of a second date with the Brisbane fighter.

Privately, the Horn camp are becoming sceptical that Pacquiao would want to face the younger Horn for a second time, nor do they believe he would appreciate another fight in Australia, where he felt he was harshly treated by judges.

Horn hardly appears to be Pacquiao's chief concern. In recent days, he has been seen rallying troops in the city of Marawi, where Filipino soldiers are in the midst of a battle with Islamic militants that have taken hostages and torched buildings.

More than 400 lives have already been lost, including 70 government troops and 40 civilians. These are the kind of deeply serious issues Pacquiao faces in his life outside the ring, which was precisely why his trainer Freddie Roach doubted he could commit to both sport and politics.

Pacquiao's camp are convinced they have finished their contractual arrangement with Bob Arum's Top Rank, with Arum also having the US rights to promote Horn, who he signed up after seeing him fight on the Duco Events card in New Zealand.

But it's likely Pacquiao will remain with Arum for what could be the final fight of his career, although he could yet retire for the second time after the 38-year-old showed his age in the ring against the hungrier, bigger, more aggressive Horn.

If that was the case, Horn could end up meeting 27-year-old Spence, who has also signalled his intention to fight before the clock ticks over to 2018. The London Olympian is 22-0 (19 KOs) as a professional and is rated as the second best welterweight in the world by The Ring magazine.

Horn put the issue under Spence's nose on Instagram, writing: 'Lets fight, let's unify, their will be a man down'. Arum wants to showcase Horn in America and if that fight was to come to fruition, it would likely be held in Las Vegas or New York.

Horn had a purse guarantee of $2 million for a Pacquiao rematch but with that now in genuine doubt, he remains keen to try and cash in on his fame and form while he is riding high on his historic victory.

Spence would, like Pacquiao, be the favourite for that fight but Horn has quickly gained respect as a fighter that will take on anyone in the division.

After a break in America and back home in Queensland, Horn will return to the gym next week for the first time since his gruelling preparations against Pacquiao.