STOCKHOLM (Reuters) - Newly available data suggests that China is the world’s second-biggest arms producer, behind the United States and ahead of Russia, a leading conflict and armaments think-tank said on Monday.

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A lack of transparency means the Stockholm International Peace Research Institute has excluded China from its annual global rankings of arms makers, but it said credible financial information had become available for four major companies.

The data, covering the period from 2015 to 2017, allowed it to compile what it called the most comprehensive picture of Chinese companies’ weapons production to date.

“With the increase of available data on these companies, it is now possible to develop reasonably reliable estimates of the scale of the Chinese arms industry,” the institute said.

The four companies had combined estimated arms sales of $54.1 billion for 2017, it said, which would put them among the top 20 arms producers in the world.

“Three of the companies would be ranked in the top 10.”

Total U.S. arms sales in 2017 were $226.6 billion, and in Russia, $37.7 billion, according to the think tank’s Top 100 list for that year.

Aircraft and avionics group Aviation Industry Corporation of China (AVIC) would rank as the sixth largest arms producer, with estimated 2017 sales of $20.1, while land systems-focused China North Industries Group Corporation (NORINCO) would place eighth with an estimated $17.2 billion in sales, the institute said.

The other two companies it looked at, China Electronics Technology Group Corporation (CETC) and China South Industries Group Corporation (CSGC), had estimated sales of $12.2 billion and $4.6 billion, respectively, a spokesperson for the institute said.

China’s foreign and defense ministries did not respond to faxed requests for comment during a public holiday. Calls to AVIC, NORINCO and CSGC went unanswered and China Electronics Technology Group (CETC) declined to comment.

The Sweden-based think tank has said global expenditure in 2018 hit $1.8 trillion, its highest level since the end of the Cold War, fuelled by increased spending in the United States and China.

U.S. arms sales that year were $246 billion, Russia’s were $36.2 billion and the United Kingdom had $35.1 billion in sales, it said.