This paper studies the leader-following consensus for a class of nonlinear singular multi-agent systems. Unlike the related works, the case that states of agents are unavailable and communication channel among agents may be subjected to frequently attacks is considered, where agents can only communicate with their neighbors on some disconnected time intervals. Based on graph theory, singular system theory and multi-Lyapunov function approach, the observer-based intermittent feedback control protocol and sufficient conditions are given to ensure the existence and uniqueness of solutions and consensus of the considered systems. Besides, a description of state jumps of the systems at the instants of communication changes is presented. Finally, a numerical example is given to illustrate the feasibility of the result.