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, according to Reuters.

Iranian weapons continue to pour into Syria from Iraq but also increasingly along other routes, including via Turkey and Lebanon, in violation of a UN 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 on the ground in Syria in support of Assad's forces, envoys say.

The Syrian conflict started out two years ago as a pro-democracy movement. Some 70,000 people have been killed and more than 1 million refugees have fled the violence.

Much of the weaponry going to Syria now, diplomats say, continues to be shipped to Iran through Iraqi airspace and overland through Iraq, despite Baghdad's repeated promises to put a stop to Iranian arms supplies to Assad in violation of a UN arms embargo on Tehran over its nuclear program.

"The Iranians really are supporting massively the regime," a senior Western diplomat said this week. "They have been increasing their support for the last three, four months through Iraq's airspace and now trucks. And the Iraqis really are looking the other way."

"They (Iran) are playing now a crucial role," the senior diplomat said, adding that Hezbollah was "hardly hiding the support it's giving to the (Syrian) regime."

He added that the Syrian civil war was becoming "more and more sectarian," with Sunnis - an increasing number of whom come from Iraq - battling Shi'ites and members of Assad's Alawite sect, an offshoot of Shi'ite Islam.