The Eurosatory arms show gets under way in France this week, featuring more than 1,000 exhibitors from 70 countries.
Human Rights Watch said it objected to the participation of Russian arms supplier Rosoboronexport at the event.
Jean-Marie Fardeau, French director for Human Rights Watch, said it's duplicitous for the French government to both condemn the Syrian government for the ongoing bloodshed and host one of Syria's largest arms suppliers.
"It's not acceptable to do business as usual with a company arming a government engaged in atrocities against its people," he said in a statement.
The rights organization says that providing weapons to Syria while the government there is suspected of committing crimes against humanity may by a violation of international law.
U.N. officials last week said they suspected Syria had committed atrocities against its people during a series of massacres the government in Damascus blames on terrorists.
Russia and China are among the leading critics of formal action against Syria at the U.N. Security Council.
British Foreign Secretary William Hague told Sky News he would continue to pressure Russian officials to get in line behind the peace plan brokered by former U.N. Secretary-General Kofi Annan.
"Every other solution of any kind to the Syrian crisis involves a lot more deaths," he said.