One parameter group of diffeomorphisms

Question

If $\phi_t : {\bf R} \rightarrow Diff(M)$ is a $continuous$ application of the real numbers in the diffeomorphims of a manifold $M$ and if we have:

$$\phi_{t+t'}= \phi_t \circ \phi_{t'}$$

then have we:

$$\phi_t =e^{tX}$$

where $X$ is a vector field of $M$?