Manchester United manager Jose Mourinho has again vented against the spending power of clubs like Manchester City by claiming: "There is no financial fair play, there is nothing, you do what you want."

Mourinho has cut a frustrated figure as City run clear in the Premier League title race, complaining that Pep Guardiola can "buy full-backs for the price of strikers".

United have signed Paul Pogba, Romelu Lukaku, Zlatan Ibrahimovic, Nemanja Matic, Victor Lindelof, Henrikh Mkhitaryan and Eric Bailly since Mourinho replaced Louis van Gaal last year, but they trail City by 15 points after 20 matches.

While giving some credit to Guardiola and City, Mourinho believes they are operating on a different financial level, even as United top the Forbes list of the most valuable football clubs in the world ahead of Barcelona, Real Madrid, Bayern and their fierce rivals.

"The club invested a lot of money," Mourinho said. "The problem is not the money we invest, the problem is the money the others invest and it is the problem that others with better squads, with better stability, with more options, they keep investing.

"That's the only problem because, for example, we bought a striker in the summer – a very good striker [Lukaku] – but that good striker was to replace Zlatan. It was not to be him and Zlatan in the best of their form from day one.

"We signed Lindelof. We didn't have Marcos Rojo [at the start of the season]. We had problems, so when we are signing players in the last two years, it's to replace people.

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"You think the club can put here now £600million and let's buy six players of £100million? The club cannot do that. I cannot expect the club to do that, so it's not critical and you can see how the market is, especially with the top teams.

"But without taking any credit away from Manchester City and Pep and his staff and the players, they obviously have lots of credit in what they are doing.

"But Pep arrives, he has the goalkeeper of England [Joe Hart], he doesn't like him so he buys the goalkeeper of Barcelona [Claudio Bravo], he doesn't like him so he buys another one [Ederson]. Now he likes.

"He has [Pablo] Zabaleta and [Aleksandar] Kolarov - two very good players but more than 30 years old. He wants to replace, he doesn't replace with two, he replaces with three. One from Tottenham [Kyle Walker], one from Monaco [Benjamin Mendy] and one from Real Madrid [Danilo] as an example. Can we buy six or seven players at the same time? Can we invest £600-700million? No. So it's difficult.

"I think in recent years, the market is going in such a direction, or you belong to one of these clubs where there is no limit and you just buy what you want and there is no limit. There is no financial fair play, there is nothing, you do what you want, or it's hard.

"Is it possible [to compete]? Yes, it's possible and sometimes if you don't have that financial - I don't say potential because potentially we have that, as a club - but not that profile of club where there are no limits and the only thing that matters is to get the best. Even with that, there is only one way which is patience and calm and time."

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Mourinho continued: "You have to believe in the work and in the time and not stop investing. But some clubs invest with limits and some clubs they invest with a little bit more balance than others that go absolutely strong in the market.

"What Paris Saint-Germain did this season with Neymar and [Kylian] Mbappe - they get two of probably the four best attacking players in the world. They get two at the same time and then players like [Angel] Di Maria, [Julian] Draxler, [Javier] Pastore, Lucas [Moura], they are second choices.

"So money makes a difference and I remember my times from my first Chelsea period where everybody was saying that the money was making a difference, so I don't think it changes. I think money makes a difference but I also ask never to misinterpret my words against my club or say that my club 'doesn't support', my club 'doesn't want to spend', because my club spends."