Ricciardo, whose frustration with the situation at Red Bull is beginning to show, earlier told reporters that Ferrari's apparent interest for 2016 is "nice" and "a compliment".
But he also acknowledged that he is under firm contract.
"I don't have experience with contracts," said the Australian, "but I would never rule anything out completely."
Dr Helmut Marko, however, has said Ricciardo's deal is "bulletproof", and team boss Horner also says the 25-year-old is definitely staying put.
"There is no risk or discussion of him being anywhere else other than Red Bull for a minimum of three years," said the Briton.
But Ricciardo said he no longer believes Red Bull can catch up with the frontrunners in 2015, even though he thinks that with a better engine, the RB11 would be "in the ballpark" with Mercedes and Ferrari.
He is less convinced that Renault's hopes of improving its struggling engine can bear fruit this year.
So while Red Bull has car improvements ready to run at Silverstone, Ricciardo said: "In terms of the power, we may find small things over the year, but I think it is what it is for this season.
"But for the car, we have a little bit coming for Silverstone which seems like a promising update. I think Silverstone will be better," he added.
|Ricciardo looking to get out from under the Renault boat anchor|
(GMM) As F1 heads to Silverstone, all eyes are on Ferrari.
And that is not because the Maranello team expects to beat Mercedes at the British grand prix. In fact, the red-colored team is set to use a different approach this weekend.
After often looking a match for Mercedes in Friday practice recently, boss Maurizio Arrivabene has concluded that the German team is obviously running routinely higher fuel loads.
"I have told my engineers 'If we want to see the true picture on a Friday, we have to use a comparable amount of fuel'," he said.
In fact, the big talking point heading into Silverstone is Kimi Raikkonen, and whether the struggling Finn is set to keep his seat in 2016.
Most insiders are now coming to the consensus that it is unlikely.
"The question is whether Kimi is better value for money than say a Hulkenberg or a Bottas," said former F1 driver David Coulthard.
Another obvious candidate is Daniel Ricciardo, who let his steadily increasing frustration with the situation at Red Bull show recently in Canada.
But Dr Helmut Marko insists the Australian has a "bulletproof" contract, and perhaps unlike Williams with Valtteri Bottas, it is unlikely Red Bull will be tempted if Ferrari knocks on the door to buy out the deal with millions.
Arrivabene, however, appears to want to chat to Ricciardo anyway.
"Perhaps he does not have my number," the charismatic Italian told La Gazzetta dello Sport last week.
For now, Ferrari is happy to keep the speculation at a distance, as spokesman Alberto Antonini on Tuesday said the rumors about Raikkonen are simply something "nice to read on the beach".
"It's been the norm at this point in the season to talk about a bit of everything, but the truth of the matter is that there are two seats and, at the moment, they are both taken," he said.
"We are sure many a driver would love to drive for Ferrari, but at the moment, the situation is clear — we have two nominated race drivers," said the Ferrari official.
But Ricciardo's interest may already have been piqued, particularly as he admitted at a Red Bull event on Tuesday that boss Dietrich Mateschitz's quit threats in Austria recently have made him nervous about the future.
More than that, however, "Not talking about contracts or anything, what I want is to win", said the 25-year-old. "A lot of my frustration is about that.
"Obviously with contracts, it is not as easy as saying 'Yeah I will go to Ferrari'. There are things in place, and right now it would be – from my understanding of contracts – it is not likely that I can get out.
"But at the same time, I really feel that Red Bull can make a change for next year and can get back up the front. I still think we have good people in the team," Ricciardo added.