Treasury officials have targeted child benefit payments for the middle classes under plans for sweeping cuts to bring the £164bn deficit under control.

The cuts are among proposals being presented to the new Conservative and Liberal Treasury team as it emerged that new policy commitments in the coalition agreement may have added up to £10bn to the government's spending bill.

The Guardian has learned that the Treasury has for months been drawing up plans for cuts of more than 15% in all departments other than the NHS and international development, where spending has been ring-fenced.

The Treasury is waiting for a report from the new Office of Budget Responsibility, chaired by Sir Alan Budd, before deciding exactly where the cuts will come, as well as a possible rise in VAT.

Until now the focus has been on Whitehall departmental budgets, as opposed to the separately accounted welfare bill. But government sources confirmed that ministers will also be looking at benefit rules.

The Treasury had already drawn up plans for Labour to reduce benefits for the middle class, and one source said child benefit for the middle class would be a prime candidate for review, adding: "Welfare for the wealthy is always on the Treasury hit list."

After the jokes and bonhomie of Wednesday's press conference in Downing Street, the new government struck a more sober tone as the chancellor, George Osborne, told the first meeting of the Liberal-Conservative cabinet that the need to cut the deficit will overshadow everything. Ministers tried to calm the immediate centre-right euphoria by warning that their departments would have to look at difficult spending cuts.

In an effort to generate an atmosphere of shared austerity, Cameron today ordered ministers to take a 5% pay cut.

The culture secretary, Jeremy Hunt, suggested that the Olympics budget might not be sacrosanct, and the health secretary, Andrew Lansley, said he was looking at efficiency savings of more than the £20bn proposed by the outgoing government.

The news came as Cameron announced 29 further appointments, including six Liberal Democrat ministers of state, but only five women – including two peers.

A smattering of Tory MPs expressed concern over the concept of Lib-Con coalition. One Liberal Democrat MP, John Pugh, for Southport, also warned that he was "very concerned" about being tied to savage Tory cuts.

He urged colleagues to pursue "a distinctive path" to ensure the poor did not suffer when spending cuts are speeded up, as promised by Osborne. "I think there is a danger that Liberal Democrats fall into agreeing with decisions made by the 'Sir Humphreys' in the civil service – and George Osborne."

Michael Fallon, on the Treasury select committee, became the first senior Tory to advocate cutting welfare for the better-off to spread the load of bringing the deficit down.

He said "If you don't tackle that side of it and you have some protected areas then you are inevitably driven into much deeper cuts in the remaining departmental spending areas than you would otherwise like to see. If you are going to spread the pain, then you do need to look at benefit eligibility as well."

He also raised the question of whether the Liberal Democrats will have the appetite to look at that "bit of public expenditure that is the benefits system and the eligibility of benefits and the way in which they are indexed each year and so on".

In a potentially precarious balancing act David Cameron appointed Iain Duncan Smith as work and pensions secretary with the Liberal Democrat left-winger Steve Webb in charge of pensions. Webb ruled out any cuts to benefits during the election campaign.

But in the joint accord agreed by the two parties, and published yesterday, the two parties agreed to "end all existing welfare to work programmes and to create a single welfare to work programme to help all unemployed people back to work".

It also agreed that receipt of benefits for those able to work should be conditional on "willingness to work", a tough phrase that may or may not involve any specific new policy to tighten conditionality.

Referring to the annual welfare bill, Robert Chote, director of the Institute for Fiscal Studies, said: "I think it is certainly one of the areas where you're likely to see more action now we're this side of the election than the parties were willing to talk about beforehand. In thinking about the repair job in the public finances there are really three areas – tax increases, cuts in spending on public services and cuts to the social security budget and it would be surprising to see action on two of those and not on the third."

The pressure to find cuts may have been increased by the agreement reached by the two parties. Research conducted by Credit Suisse suggested that the two parties had added £10bn of extra spending proposals, including extra spending on disadvantaged children.

Liberal Democrats have accepted a Conservative "ring-fence" to guarantee real-terms increases – that is, rises above inflation – in NHS funding every year.

These had been criticised by the new business secretary, Vince Cable, during the election campaign. He said they would lead to deeper cuts elsewhere.

Lansley said Lib Dems had accepted the need for real terms increases and acknowledged that "those will have an impact on other departments". But he warned: "If we protect the real value [of the NHS budget] relative to the level of inflation in the economy as a whole, it will not protect the NHS from the need to secure efficiency savings and to control pay and prices in the NHS."