MOSCOW (AP) -- A Russian transport helicopter was shot down in Syria on Monday but the fate of the five people onboard was not immediately known, Russia's Defense Ministry said.
The Mi-8 helicopter was shot down in Idlib province while returning to the Russian air base on Syria's coast after delivering humanitarian goods to the city of Aleppo, the ministry said in a statement.
The helicopter had three crew members and two officers deployed with the Russian center at the Hemeimeem air base on the Syrian coast.
If the Russian crew and officers are confirmed killed, it would be the deadliest single incident for the Russian military since September, when Moscow began carrying out airstrikes in Syria in support of Syrian President Bashar Assad's forces.
There was no immediate claim of responsibility for the helicopter incident. Idlib province has a strong presence of both fighters for the al-Qaida branch in Syria known as the Nusra Front and rebels. The group announced last week that it was changing its name and relinquishing ties with al-Qaida in an attempt to undermine a potential U.S. and Russian air campaign against its fighters.
The group is part of a coalition of insurgent groups called Jaish al-Fateh, or Army of Conquest, which has captured most of Idlib.
In July, two Russian airmen were killed in the central Homs province when their Mi-25 helicopter was shot down by what the Defense Ministry said were Islamic State fighters.
An Mi-28N helicopter gunship crashed near Homs in April, killing both crew members, but the Russian military said there was no evidence it came under fire.
A Russian warplane was shot down by a Turkey along the Syrian border in November, and one of the two pilots was shot and killed from the ground after ejecting.
Earlier on Monday, a Syrian military official said that government forces repelled an attack by insurgents that was an attempt to break the siege imposed on rebel-held parts of the northern city of Aleppo.
The development came a day after Syrian rebels launched the offensive to break up the government's siege of eastern, rebel-held part of the city.
The U.N. estimates some 300,000 people are still trapped in the rebel section of Aleppo, with dwindling food and medical supplies. The U.N.'s special envoy to Syria, Staffan de Mistura warned on Friday that basic supplies in eastern Aleppo could run out in three weeks.
Opposition activists said intense fighting was still ongoing in Aleppo on Monday. The Syrian military official, who spoke on condition of anonymity in line with regulations, did not elaborate.