British and European foreign ministers are preparing to defy Washington at talks over an arms trade treaty, amid fears the US will use its diplomatic clout to water down proposals for the first comprehensive laws governing weapons sales.

More than 150 countries have sent delegations to the UN for the special month-long session that begins on Monday, with many hoping that 15 years after the idea of an arms trade treaty was first mooted by Nobel peace prize winners, and seven years after the UK took an unexpected lead on the issue, the UN is now close to an agreement that could transform the $1tn arms industry.

A draft of the treaty, agreed this year, states that governments must not approve arms sales to countries where there is a "substantial risk of a serious violation" of human rights. If the draft treaty was in place now, it would prevent Russia from sending arms to Syria.

But the US wants the wording of this key component of the draft treaty changed to say governments need only "consider" factors such as human rights records before authorising weapon sales.

In a statement issued on the eve of the talks, the foreign secretary, William Hague, and his counterparts in France, Germany and Sweden said they would be pushing for a treaty that included "strong provisions on human rights". They said the UN had a responsibility to agree a "robust, effective and legally binding arms trade treaty. Now is the time for us to deliver."

As well as reservations about the wording in the treaty, the US believes it will be too difficult to include ammunition in any final document. "The US has been supportive of the treaty, but it does have some issues with it," said one European diplomat who will be involved in the talks. "Parts of the US system don't favour the inclusion of ammunition … but we will be arguing hard for this. We cannot promise what the outcome will be. We can promise what we are fighting for."

Writing for the Guardian, Alan Duncan, the minister for international development, and the Foreign Office minister Alistair Burt said it was scandalous that there was currently no international regulation of the arms trade. They said it was "crucial that a treaty includes strong provisions on human rights and international humanitarian law".

Both ministers will be travelling to New York to take part in the talks, and it is thought the deputy prime minister, Nick Clegg, may go if negotiations stall.

The former foreign secretary Jack Straw put the UK at the forefront of the countries campaigning for a tough arms trade treaty, and Duncan has said the illegal arms trade is now the single biggest obstacle to providing development. It is estimated that 750,000 people die each year as a result of violence enabled by illegal gun sales.

The UK government says the treaty must "include all conventional weapons, from fighter planes to rifles, bombs to bullets. Arms brokering must be controlled and corrupt practitioners prosecuted."

Duncan said: "About two-thirds of countries least likely to achieve global development goals are in the midst of, or emerging from, conflict. We have seen the shocking pictures of child soldiers. These boys, who would be hardly out of primary school in this country, are trained for combat. Young girls are taken to work as slaves, whilst others are raped and abandoned. The groups who perpetrate these atrocities would not be able to operate without access to a constant source of weapons and ammunition."

Those countries that have expressed the biggest reservations about the treaty include Russia, China and Zimbabwe.

Thomas Countryman, an assistant secretary of state in Washington, said in April that the US believed it was "hugely impractical" to include ammunition. "We do not want something that would make legitimate international arms trade more cumbersome than the hurdles United States exporters already face," he said.

Anna Macdonald, the control arms campaign manager at Oxfam, said: "The United States is a really key player in these negotiations. It is very positive that they are [treaty] supporters, but if ammunition is excluded, it really weakens the treaty.

"Ammunition is the fuel of conflict. The treaty must unambiguously require governments to not authorise any arms transfers where there is a clear risk they will end up fuelling poverty, conflict and human rights abuses."

Brian Wood, of Amnesty International, said: "US negotiators have tended to adopt a minimalist approach to the rules needed in the treaty compared to most of their strategic allies. They have been accused of trying to appease Moscow, Beijing and other players in the talks.

"Just listing principles to take into consideration is more or less saying you can feel free to ignore those principles if you want to. And if this is how the criteria on international human rights and humanitarian law will be reflected in the treaty, it will simply not prevent arms fuelling atrocities and abuses."