UNITED NATIONS (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.

Syria's President Bashar al-Assad (R) meets Iran's Supreme National Security Council Secretary Saeed Jalili in Damascus February 3, 2013, in this handout photograph released by Syria's national news agency SANA. REUTERS/Sana

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 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.

A Western intelligence report seen by Reuters in September said Iran was using civilian aircraft to fly military personnel and large quantities of weapons across Iraqi airspace to aid Assad. Iraq denied that report but later made a point of inspecting an Iran-bound flight that it said had no arms on board.

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 U.N. 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.

Ali al-Moussawi, Iraqi Prime Minister Nuri al-Maliki’s media adviser, strongly denied the allegations, saying on Wednesday: “No, such a thing never happened. Weapons did not and will not be transferred from Iran to Syria through Iraq, whether by land or by air.”

Russia, diplomats said, also remained a key arms supplier for Assad. Unlike Iran, neither Syria nor Russia is subject to a U.N. ban on arms trade and are therefore not in violation of any U.N. rules when conducting weapons commerce. But accepting Iranian arms would be a violation of the U.N. Iran sanctions.

Assad’s ally Russia criticizes U.S., European and Gulf Arab governments for their aid to Syrian rebels seeking to topple Assad.

Russia has said repeatedly that its military support for Syria includes anti-missile air defense systems but no attack weapons such as helicopters.

Moscow says it is not wedded to Assad but that the rebels and government should talk and Assad’s departure should not be a condition for a deal as the opposition and its supporters insist. Along with China, it has used its Security Council veto to block punitive U.N. measures against Syria’s government.

ARMS SUPPLIES VIA TURKEY AND LEBANON?

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.”

“Unfortunately the situation in Syria and the whole Middle East region is becoming more and more delicate and risky because of foreign interference and funneling of arms to the extremist groups,” he said, repeating that Tehran wanted to end the conflict through dialogue between the government and opposition.

Syria’s U.N. ambassador, Bashar Ja’afari, did not respond immediately to a request for comment.

The diplomats cited by Reuters made clear that the principal delivery route for arms to Syria still went through Iraq, despite the existence of alternative supply channels such as Turkish airspace. They also said that Iran Air and Mahan Air were well-known violators of the Iranian arms embargo.

Iran Air and Mahan Air were both mentioned in the intelligence report on Iranian arms shipments to Syria seen by Reuters in September. The U.S. Treasury Department has blacklisted Iran Air, Mahan Air and Yas Air for supporting the Islamic Revolutionary Guards Corps.

One Western diplomat cited intelligence reports from his country that a new avenue for sending arms to Syria went on occasion through Turkish airspace to Beirut and from there to Syria by truck. There was no suggestion, he said, that Turkish officials were aware of the illicit arms shipments.

Once in Syria, he said, the arms were distributed to government forces and allied militia, including Hezbollah.

“The equipment being transferred by both companies (Iran and Mahan Air) ... ranges from communications equipment to light arms and advanced strategic weapons, some of which are being used devastatingly by Hezbollah and the Syrian regime against the Syrian people,” said the Western intelligence report.

“The more sophisticated gear includes parts for various hardware such as unmanned aerial vehicles (UAVs), shore-to-sea missiles and surface-to-surface ballistic missiles (SSMs),” the report said. “Other weapons are being used by Syrian security forces, pro-Assad shabbiha militiamen, and Lebanese Hezbollah.”

There are about 5 tons of arms per flight, which are occurring on a near weekly basis, hidden in the bottom of the planes’ fuselages, the report said, adding that arms cargo was removed separately after civilian cargo was unloaded.

Other Western officials confirmed the findings in the report.

A Turkish diplomatic source denied the allegation. “This is a very sensitive matter for Turkey, and we are very certain that this is baseless,” the source told Reuters.

Turkey has intercepted Iranian arms shipments in the past and reported them to the U.N. Security Council’s sanctions committee. Ankara’s aggressive campaign to stamp out Iranian arms smuggling via its airspace, Western diplomats say, was what led Iran to begin using Iraqi airspace instead.

Lebanon’s U.N. ambassador, Nawaf Salam, said he was not in a position to comment. An official at Beirut’s airport who requested anonymity rejected the allegations of clandestine Iranian shipments going to Syria via Beirut airport.

Lebanon has had a complicated relationship with neighboring Syria. Its population is deeply divided over the conflict. U.N. chief Ban Ki-moon last week urged Lebanon, which is hosting hundreds of thousands of Syrian refugees, to remain neutral.