The alliance of countries known as the Friends of Syria, says President Bashar al-Assad has no role in the future of the nation.
The 11 countries also pledged to increase support for the opposition.
Their statement comes as US Secretary of State John Kerry threatened to increase support for Syria's rebels if Mr Assad does not take part in peace talks planned for June.
Mr Kerry said the proposed talks backed by Mr Assad's main ally Russia, offer the only chance for a political solution to the two-year-old civil war, the ABC reports.
Mr Kerry has been meeting European and Arab allies in Jordan to coordinate their next step on Syria.
Meanwhile, Britain and France have urged other European Union countries to designate as a terrorist organisation the military wing of the Shi'ite Islamist group, Hezbollah.
Hezbollah is fighting on Mr Assad's side in the civil war.
French Foreign Minister Laurent Fabius said his government acted because of Hezbollah's siding with Mr Assad's forces in the civil war, the BBC reports.
The move comes after Germany changed its stance, having long been against anything that could destabilise Lebanon.
