Human Rights Watch is calling for a boycott of new arms contracts with companies supplying the Syrian regime as the nearly 15-month government crackdown forced the country to the brink of a civil war.

The call comes after U.N. envoy Kofi Annan said Syria is "at a turning point" and that "the specter of all-out civil war, with a worry sectarian dimension, grows by the day."

Human Rights Watch on Sunday singled out Rosoboronexport, a Russian state-owned weapons supplier, for sending arms to the Syrian regime.

"The (U.N.) Security Council should impose a mandatory international arms embargo on Syria, and Russia and China should not block it," the group's executive director, Kenneth Roth, said in a statement. He accused the Syrian regime of crimes against humanity and said "other governments and companies around the world should use whatever leverage they have to stop further arms supplies that could contribute to these crimes."

Rosoboronexport did not immediately respond to the rights group's allegations.