Washington called on the Syrian regime Thursday to quickly declare the scope and size of its chemical weapons stockpile as US Secretary of State John Kerry arrived in Geneva for high-stakes talks.

Backed by a large team of US experts, Kerry was to hold talks later in the day with Russian counterpart Sergei Lavrov on Moscow's proposal to secure and destroy Syria's chemical weapons. "It's doable, but difficult," a US official told reporters accompanying the top US diplomat.