China, India and Austria have condemned a US decision to impose sanctions on nine firms which it believes have supplied Iran with military equipment.
Austria says the sale of rifles by Steyr-Mannlicher was entirely legal
China, which is home to six of the firms concerned, has demanded that the US State Department lift the sanctions.
The Austrian interior ministry defended the sale of about 800 sniper rifles to Iran by an Austrian arms manufacturer as "unimpeachable".
India criticised the sanctions imposed on two of its firms as unjustifiable.
The measures - which will remain in place for two years - ban the firms from trading with the US government and prevent them obtaining export licences needed to buy certain kinds of technology from US companies.
Announcing the action, the US State Department said it was based on "credible evidence" and described the firms involved as "serial offenders".
It was taken under the US Iran Non-proliferation Act, which aims to deter international support for Iran's nuclear, chemical and missile-based weapons programmes.
The US and EU suspect Iran of pursuing nuclear weapons, but Tehran says its nuclear programme is for civilian energy use.
China's foreign ministry said the US must reverse its "mistaken action" against six Chinese firms, warning it could damage cooperation between the countries.
"We are strongly dissatisfied with and firmly opposed to the US government sanctioning Chinese companies," it said.
"The Chinese government has always adopted a responsible attitude on the anti-proliferation issue."
The US fears Iran wants nuclear and other weapons programmes
Indian foreign ministry spokesman Navtej Sarna said neither of the two India-based companies accused of supplying chemical weapons material had breached national laws or regulations.
He said Delhi was working with the US to prevent weapons proliferation.
Austrian interior ministry spokesman Johannes Rauch said the sale of sniper rifles to Iran by arms manufacturer Steyr-Mannlicher had been approved and controlled by the government before it was completed in August.
The guns had been supplied for the use of Iran's elite anti-drugs operatives, he said.
State Department spokesman Adam Ereli acknowledged that cooperation from Austria had been excellent, suggesting the matter may soon be resolved, the AFP news agency says.
The Chinese companies involved are: China Aerotechnology Import Export Corporation, China North Industries Corporation, Zibo Chemet Equipment Company, Hongdu Aviation Industry Group, Ounion International Economic and Technical Cooperative Limited and the Limmt Metallurgy and Minerals Company.
The two Indian companies are Sabero Organics Chemical and Sandhya Organics Chemical.