The chairman of the Iranian parliament’s National Security and Foreign Policy Commission, Alaeddin Boroujerdi, right, meets with Syria’s parliament speaker, Mohammad al-Laham, in Damascus on Oct. 14, 2015. (Omar Sanadiki/Reuters)

Hundreds of Iranian soldiers have massed in northern Syria in recent days and are poised to back Syrian forces in a ground offensive under cover of Russian airstrikes, a senior U.S. defense official and Syrian activists said Wednesday.

The element from Iran’s elite Quds Force of the Revolutionary Guard Corps has gathered near the Syrian city of Aleppo, said the defense official, who spoke on the condition of anonymity to discuss intelligence matters.

The number of Iranian soldiers is in the hundreds, rather than thousands, as some reports have indicated, he said.

Syrian officials denied the claims, however, while some Syrian rebels said that militia fighters from Iraq but backed by Iran were being deployed in the area.

Iran has long had commanders and military advisers on the battlefield in Syria. But the deployment would show a deepening involvement in the four-year civil war in support of President Bashar al-Assad as Russia steps up its military intervention, assisting Syrian ground forces with two weeks of airstrikes.

Moscow says its strikes have mainly targeted Islamic State militants, but U.S. officials and Syrian rebels say the Russian strikes have largely hit other groups fighting Assad and are aimed at propping up his government.

The defense official said the Iranian forces and Syrian troops, backed by Russian airstrikes, are expected to begin an operation near Aleppo imminently.

A salvo of 26 Russian cruise missiles launched at targets in the area last week from nearly 1,000 miles away in the Caspian Sea probably were fired to destroy targets before the upcoming offensive, the U.S. defense official said.

The Islamic State has little presence in Aleppo but made gains around the city last week after the Russian cruise-missile strikes on other rebel groups — raising questions over whether any offensive will target territory held by the Islamist militants.

The Pentagon has tracked ­Russian ships that fired the ­Kalibr-type cruise missiles from the Caspian Sea last week, and the ships have been resupplied with missiles and are ready to fire another salvo, the official said.

The Syrian Observatory for Human Rights, a Syrian activist network based in Britain, said Iranian forces also had been deployed to Syria’s central Hama province and to the mountains surrounding Latakia, the coastal heartland of Assad’s Alawite sect, an offshoot of Shiite Islam.

The alliance between Russia and the regime of Bashar al-Assad goes back decades. Here's a bit of historical context that explains why Russia is fighting to prop up its closest ally in the Middle East. (Ishaan Tharoor and Jason Aldag/The Washington Post)

Troops have arrived through Hmaymim airport in Latakia, the group said, using the former name for the city’s Assad International Airport.

Syrian Information Minister Omran al-Zoubi dismissed the reports of a buildup of Iranian forces.

Iranian military personnel in the country play only a consultative role, he said, according to comments carried by Iran’s Press TV.

Islam Alloush, a spokesman for the Syrian rebel group known as the Islamic Army, said Shiite fighters from Iranian-backed militias in Iraq had been deployed.

Three senior Iranian commanders have been killed in Syria in the past week, pointing toward an increased involvement. The most senior, Hossein Hamdani, was reported killed on the outskirts of Aleppo on Friday. He was lauded in the Iranian press as having played a major role in coordinating volunteer militia forces in Syria with the Syrian army.

Just three days later, two other senior officers with Iran’s Revolutionary Guard Corps were killed. Hezbollah, the Iranian-backed Shiite paramilitary group also backing Assad on the battlefield, has sent reinforcements to Syria in recent days, the Associated Press reported.

Photographs circulated online this week purported to show Maj. Gen. Qasem Soleimani, the Quds Force commander, addressing forces in Syria. He has appeared frequently on the battlefield in Iraq, where he is overseeing the support of Iranian-backed Shiite militias fighting the Islamic State.

He was reported to have visited Moscow in July to meet with President Vladimir Putin.

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