WASHINGTON—Venezuelan President Hugo Chávez and his state-oil company are providing vital energy support to Syrian President Bashar al-Assad and conducting business with Syrian firms blacklisted by Washington and Brussels, according to documents relating to the deals.

That puts Venezuela alongside Russia and Iran in an informal bloc of nations working to stymie the West's efforts to topple Mr. Assad and break up his military alliance with Iran, U.S. officials and Syrian activists say.

The support centers on diesel fuel sent by Venezuela to Syria, shipments that officials from both sides of the trade have publicly confirmed. But the deals are structured to bring other benefits, including shielding Syria's dwindling foreign-exchange reserves, the documents show.

Oil company Petróleos de Venezuela SA, or PdVSA, is processing its fourth major shipment of diesel fuel to Syria in the past eight months, according to Venezuelan and Syrian corporate documents reviewed by The Wall Street Journal.

Venezuela also is helping Damascus evade Western sanctions by purchasing Syrian energy and conducting trade with two firms, the Commercial Bank of Syria and Sytrol, the state's oil-marketing firm, according to the documents. Both companies are on U.S. and European Union sanctions lists.