The statement of support for the trade deal came just ahead of the opening of the Group of Eight Summit among the leaders of leading industrial nations: The U.S., Britain, Germany, France, Italy, Japan, Canada and Russia, plus the European Union.
Host Cameron greeted the arriving leaders against a backdrop of green grass and lake views at the Lough Erne golf resort near the small town of Enniskillen.
Later Monday over a working dinner, Russian President Vladimir Putin and Russia's support for the Syrian government will be a topic for discussion.
Cameron said he wanted the summit to deliver a firm plan for promoting multi-faction talks in Geneva aimed at ending the country's bloody civil war. Moscow officially shares that goal but continues to ship military aid to the army of Syrian President Bashar Assad.
Following a frosty Putin-Cameron meeting Sunday in London, the Russian leader remains opposed to plans by the West to start shipping weapons to rebels struggling to topple Assad.
Obama announced Friday that the U.S. would start sending weaponry, while Britain and France remained concerned that the firepower might end up helping anti-democratic extremists linked to Iran and Lebanon's Hezbollah militia.
"How can you allow Russia to continue to send weapons to the regime of Bashar al-Assad while the opposition gets so few weapons? " asked French President Francois Hollande. "We will have this discussion .<!p>.<!p>. Not to point the finger at Vladimir Putin, but for him to understand that it is in the interest of world peace to have this conference in Geneva."
The summit is taking place under heavy security, with some 7,000 police lining roads and checking vehicles.
A protest Monday attracted far fewer than the 2,000 expected by police. About 200 people gathered ahead of the march at a library in Enniskillen. Activists waved flags and placards for a range of causes as a heavy metal band played, including gay marriage, anti-capitalist criticism of the G-8 forum itself, and opposition to fracking.