Web service composition requires allocating suitable resources to a set of services that constitute a composite service. The problem is complicated by the fact that the availability of resources may be uncertain and change over time, and the constraints of component services may not be completely known in advance. It needs the ability to rapidly and dynamically identify suitable resources and coordinate them under various constraints. In this paper, an agent mediated coordination framework for web service composition is proposed in the context of e-supply chain management (e-SCM). Global constraints to be satisfied by the composite service are first translated into local constraints of its component services. Service brokering agents dedicated to discovery of component services interact autonomously with each others for refining the local constraints based on the candidate services' instantaneous status until a sufficiently good composition is identified. Although presented in the context of supply chain management, this framework is applicable to other problem areas where a set of services are composed as a result of a large resource repository to be searched, scheduled and coordinated, especially in a real-time fashion.