(Reuters) - Iran has significantly stepped up military support to Syrian President Bashar al-Assad in recent months, solidifying its position alongside Russia as the government's lifeline in an increasingly sectarian civil war, Western diplomats said.

Iranian weapons continue to pour into Syria from Iraq but also increasingly along other routes, including via Turkey and Lebanon, in violation of a U.N. arms embargo on Iran, Western officials told Reuters on condition of anonymity. Iraqi and Turkish officials denied the allegations.

Iran's acceleration of support for Assad suggests the Syrian war is entering a new phase in which Iran may be trying to end the battlefield stalemate by redoubling its commitment to Assad and offering Syria's increasingly isolated government a crucial lifeline, the envoys said.

It also highlights the growing sectarian nature of the conflict, diplomats say, with Iranian arms flowing to the Shi'ite militant group Hezbollah. That group is increasingly active in Syria in support of Assad's forces, envoys say.

Alireza Miryousefi, spokesman for Iran's U.N. mission, responded to a request for a comment by saying, "We believe Syria does not need any military help from Iran."

Syria's U.N. ambassador, Bashar Ja'afari, had no specific comments on the matter.