Jan. 27 (UPI) -- China is the world's second-largest arms producer, after the United States, a Swedish research center announced on Monday.

Information on China's weapons manufacturing has been unreliable in the past because of a lack of transparency, causing the Stockholm International Peace Research Institute to exclude Chinese companies from consideration until now.

For years, the United States has ranked first and Russia second in the organization's SIPRI Top 100 list of arms-producing and military services companies.

Credible data from 2015 to 2017 relating to four large Chinese companies has emerged, a SIPRI statement said Monday, allowing development of "reasonably reliable estimates of the Chinese arms industry."

Those estimates put China ahead of Russia in arms sales, behind only the United States.

The four Chinese companies combine for $54.1 billion in sales, and would place each among the top 20 armaments producers in the world.

One, Aviation Industry Corp., would rank sixth in the world with $20.1 billion in sales. Another, China North Industries Group Corp., would be the world's largest producers of land armaments systems with $17.2 billion in sales.

The SIPRI study's largest arms makers, including the Chinese companies, are, in order: Lockheed Martin, Boeing, Northrop Grumman and Raytheon, all of the United States; Britain's BAE Systems; China's Aviation Industry Corp., the United States' General Dynamics; the trans-European Airbus; France's Thales; Italy's Leonardo, and China North Industries and China Electronics Technology Group.

The survey excludes China from its list of arms sales by country, but indicates that the United States has 57.9 percent of the international arms market, followed by the United Kingdom with 9.6 percent and Russia with 7.1 percent.