Iran has boosted its military support for the Syrian government in recent months, solidifying its position alongside Russia as the government's "lifeline" as rebels seize more territory across the country, Western diplomats have said.
Iran continues to send weapons to Syria from Iraq, but is also increasingly using other routes, including Turkey and Lebanon, in violation of a UN arms embargo on Iran, Western officials told the Reuters news agency on condition of anonymity.
Iraqi and Turkish officials denied the allegations.
Iran's drastic increase in support for Syrian President Bashar al-Assad suggests the Syrian conflict is entering a new phase in which Tehran is trying to counter major advancements by Syrian opposition forces, the envoys said.
They added that Iranian arms are also flowing to Hezbollah, a Lebanese group that is increasingly active on the ground in Syria in support of Assad forces against opposition groups.
A Western intelligence report seen by Reuters in September last year said Iran was using civilian aircraft to fly military personnel and large quantities of weapons across Iraqi airspace to aid Assad, which Baghdad denied.
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."
Ali al-Moussawi, Iraqi Prime Minister Nuri al-Maliki's media adviser, strongly denied the allegations 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," he said.
Alireza Miryousefi, the first secretary of Iran's mission to the UN, told Al Jazeera's diplomatic editor, James Bays that Syria does not need any military help from Iran, and that the country welcomes initiatives for a ceasefire and a cessation of violence.
"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," Miryousefi said.
Iran is of the view that the way out of present crisis lies in strengthening a comprehensive peaceful political process under regional and international initiatives," he said adding that the Syrian opposition and government should hold a national dialogue for a "peaceful political process".
Russia also remained a key arms supplier for Assad, diplomats said. Unlike Iran, neither Syria nor Russia is subject to a UN ban on arms trade and are therefore not in violation of any UN rules when conducting weapons commerce.