Countries in the forefront of arming either side in Syria's civil war have been among the least generous when it comes to dealing with the resulting humanitarian disaster, according to a new Oxfam report.

The aid agency and advocacy group found that Russia and Qatar had committed just 3% of their fair share to the United Nations humanitarian appeal, measuring their contributions as a proportion of national income and wealth.

Russia has long been the Syrian government's main arms supplier, providing nearly half its imports in 2006-10, according to the Stockholm International Peace Research Institute

On Wednesday, Moscow launched a diplomatic assault in defence of the Syrian regime, claiming to have evidence implicating Syrian rebels in the chemical attack in Damascus on 21 August, in which hundreds were killed. Russia's move once again pits it against the UK, France and the United States, which blamed the attack squarely on Bashar Al Assad.

Meanwhile, Qatar is widely reported to be the main source of finance for weapons for the rebels, particularly jihadist groups.

France, the most vociferous supporter of the opposition in western Europe, has given less than half its fair share, the Oxfam report found.

At the other end of the scale, Kuwait has contributed more than four times its share, while Britain has given more than one and a half times what the agency estimated a proportionate contribution to the UN fund. Saudi Arabia has given nearly twice its share.

Overall, under-payers far outnumber over-payers, especially among rich countries. The US, despite being the biggest contributor in absolute terms, has given 63% of its fair share in relation to national income, Oxfam found. Japan has paid 17% of its fair share and South Korea 2%.

As a result, the £3bn Syrian humanitarian fund launched by the UN in June this year is so far only 44% funded, with days to go before a high-level donor meeting on the sidelines of the UN general assembly next week.

Oxfam's own emergency appeal, aimed at raising just over £30m, is only 39% funded, the agency said.

"Too many donor countries are not delivering the level of funds that is expected of them," said Colette Fearon, the head of Oxfam's Syria programme.

"The scale of this crisis is unprecedented and some countries must start to show their concerns to the crisis in Syria by putting their hands in their pockets. Countries such as France and Russia are failing to provide the humanitarian support that is desperately needed."

Oxfam said the funding gaps are already affecting the ability of aid organisations to respond to the crisis.

"When funding is so tight, every aid pound counts. We're seeing people go without food, shelter and water on a daily basis," said Fearon.

There are two million refugees from the Syrian conflict, half of them children, mostly in Jordan and Turkey. More than four million people are estimated to be need of urgent assistance inside Syria.

• This article was amended on 19 September. The earlier version said Oxfam's emergency appeal was aimed at raising just over £30bn.