By Darya Korsunskaya and Alexander Marrow

(Reuters) - Russia could miss its 2024 revenue target and be forced to hike business taxes if the rouble proves stronger than expected in the budget and optimistic economic assumptions fall short, analysts warned, as Moscow spends more on its war in Ukraine.

Budget plans published in September envisage Brent crude prices averaging $85 per barrel next year - more pessimistic than a Reuters poll forecast - and a Urals price of $71.3.

But Russia's Accounts Chamber, which oversees budget execution, warned on Monday there were risks the Urals price would fall below $60 in 2024-2026.

Meanwhile, the West is keen to stop Russia from circumventing its $60-per-barrel oil price cap. Washington last week imposed the first sanctions on owners of tankers carrying Russian crude priced above that level.

The government is also relying on the currency remaining weak, which - while fanning inflation and eroding people's savings - raises the rouble value of energy revenues received in dollars.

"Next year's budget is very ambitious," said Expert RA Chief Economist Anton Tabakh.

"The deficit problem has been solved by the weak rouble. If the rouble appreciates strongly then the budget will be in a difficult position."

The rouble leapt off more than 18-month lows to the dollar last week after President Vladimir Putin ordered the mandatory sale of some foreign currency revenues for certain exporters.

It now trades at around 97 per dollar, softer than the budget's average forecast for 2024 of 90.1. The currency is historically weak, however, having rarely traded above 80 before Russia's invasion of Ukraine.

Finance Minister Anton Siluanov on Monday highlighted the risks of any strengthening, saying: "A change in the exchange rate by one rouble will lead to an increase or a decrease in budget revenues (of) around 100 billion roubles."

ARTISTIC APPROACH

Revenues are expected to climb 22.3% year-on-year to 35.1 trillion roubles next year, or 19.5% of gross domestic product.

Promsvyazbank analyst Denis Popov said that was possible but warned there are risks that oil and gas revenues especially could fall short.

Russia's forecast sees economic growth of 2.3% in 2024, well above estimates of 1.1% from the International Monetary Fund and 0.5%-1.5% from the Bank of Russia.

CentroCreditBank economist Yevgeny Suvorov described the budget as an artistic approach to realising strategic goals.

"There was a task to finance military expenditures, but also ... to show a return to the budget rule framework in 2025," Suvorov said.

"For this, it was necessary to depict rapid income growth. The best tool for this is the forecast they've drawn up."

Russia is walking a budget tightrope, and lower-than-expected energy revenues or GDP growth could prove costly.

"The revenue forecast, in our opinion, looks optimistic, both from the point of view of the economic prerequisites in he economy ministry's forecasts, and the preconditions for collection," said Renaissance Capital economists Sofya Donets and Andrei Melaschenko. They predicted an income shortfall of 1 trillion roubles.

Higher non-oil-and-gas revenues have helped narrow the deficit this year, but economist Dmitry Polevoy noted that the budget anticipated them weakening in future years, which could force the central bank to keep interest rates high.

"Risks for the non-oil-and-gas deficit are clearly skewed towards higher values," said Polevoy.

The central bank, which has hiked borrowing costs by 550 basis points since July and expects elevated rates for some time, has itself noted the government's optimism, saying the finance ministry's projections envisage a higher non-oil-and-gas surplus than its own macroeconomic forecast sets out.

CHANGING TUNE

At Russia's flagship economic forum in St Petersburg in June, Siluanov said increasing expenditure was difficult, as budget spending had already increased by 1-1/2 times from 2019 to 2022. Spending in 2024 is set to be double 2019's figure.

A bigger deficit would mean higher inflation and interest rates, the finance minister said, for which the population and business would pay. Taxes could also rise.

"If we want more spending, you have to understand where the money is coming from," Siluanov said. "Money doesn't come out of thin air."

Now, even as the government outlines plans for spending to jump to 36.7 trillion roubles in 2024, he is more relaxed.

"(It's a) fine, healthy budget, the temperature is fine," Siluanov said in September.

Russia has raised taxes on oil and gas firms by around 3.6 trillion roubles ($37 billion) in 2023-2025 and imposed a windfall tax on some company profits. Analysts expect more new taxes to cover any revenue shortfalls.

"Resources are finite. Experience shows that business, rather than citizens, will be actively sheared," said Expert RA's Tabakh.

Renaissance Capital's Donets and Melaschenko said Russia could create temporary taxes, permanently increase rates of VAT, or adjust Russia's budget rule to permit more spending of energy revenues.

Deputy Finance Minister Alexei Sazanov said in September that solutions were needed in response to higher spending.

"Tax tactics are having to become adaptive," Sazanov said. "These are the realities in which we live."

University of Chicago economist Konstantin Sonin said the key unknown for Russia's public finances is how much longer it will have to budget for war.

"They could survive a year or two, maybe five years, but this is setting the whole country, the whole budget, on a very wrong path," he said. "It's certainly not sustainable in the long run."

($1 = 97.3050 roubles)

(Graphic by Sumanta Sen; Additional reporting by Mark Trevelyan; Writing by Alexander Marrow; Editing by Catherine Evans)