Washington and other allies of Bahrain's monarchy have encouraged negotiations in the strategic kingdom, home to the U.S. Navy's 5th Fleet.
Persistent clashes and arrests have so far derailed bids for talks between Bahrain's Sunni leadership and the Shiite opposition, which began an Arab Spring-inspired uprising for a greater political voice in February 2011. More than 55 people have died in the unrest.
A statement Monday by the main Shiite political factions said envoys were open to negotiations but called the government's offers for talks "vague."
Opposition demands include the release of jailed Shiite political leaders and activists.