A delegation of Iranian lawmakers arrived in Damascus on Wednesday in the build-up to a joint operation against insurgents in northwest Syria, and said US-led efforts to fight rebels had failed.
The visit, led by the chairman of the Iranian parliament's National Security and Foreign Policy Commission, Alaeddin Boroujerdi, came as Iranian troops prepared to bolster a Syrian army offensive that two senior officials told Reuters would target rebels in Aleppo.
The attack, which the officials said would be backed by Russian air strikes, underlined the growing involvement in the civil war of Syrian President Bashar al-Assad's two main allies, which has alarmed a US-led coalition opposed to the president that is bombing Islamic State (IS) militants.
"The international coalition led by America has failed in the fight against terrorism. The cooperation between Syria, Iraq, Iran, and Russia has been positive and successful," Boroujerdi was quoted as saying by Iran's state broadcaster IRIB as he arrived at Damascus airport.
The delegation was due to meet Assad, said officials.
Iran has sent thousands of troops into Syria in recent days to bolster the planned ground offensive in Aleppo, the two officials told Reuters.
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Meanwhile, a missile test announced by Iran over the weekend was an apparent violation of a UN Security Council resolution and Washington will raise it at the United Nations, the US State Department said on Tuesday.
Iran said it tested a new precision-guided ballistic missile on Sunday, signaling an apparent advance in Iranian attempts to improve the accuracy of its missile arsenal.
"We'll obviously raise this at the UNSC as we have done in previous launches," State Department spokesman Mark Toner told reporters, noting the test appeared to be a violation of UN Security resolution 1929.
He and White House spokesman Josh Earnest both said the issue was separate from a deal Iran struck in July with six world powers, which seeks to curb Tehran's atomic program in return for having sanctions against it eased.
Iran's Tasnim news agency also reported that two senior officers from Iran's Revolutionary Guards — Major General Farshad Hasounizadeh and Brigadier Hamid Mokhtarband — were killed fighting IS militants in Syria on Monday, though it did not say where.
Another senior Revolutionary Guards commander, Hossein Hamedani, was killed last week while advising the Syrian army near Aleppo.
Assad's government has been strengthened by two weeks of Russian air strikes that the Kremlin says are targeting Islamic State. The United States says they have also targeted other rebel groups.
With military support from Russian and Iran, the Syrian army is trying to drive insurgents from western areas crucial to Assad's survival.
Iranian Deputy Foreign Minister, Hossein Amir Abdollahian said on Tuesday that Tehran was working with Russia on drafting a peace plan for Syria. But Western powers, and many countries in the Middle East, say Assad must go as a precondition for peace.
Abdollahian said Syria's deputy foreign minister, Faisal Mekdad, would visit Tehran next week to discuss the peace plan.
Watch the VICE News documentary, The Battle for Aleppo: