BEIRUT — Hundreds of Iranian troops are being deployed in northern and central Syria, dramatically escalating Tehran’s involvement in the civil war as they join allied Hezbollah fighters in an ambitious offensive to wrest key areas from rebels amid Russian airstrikes.
Their arrival, a regional official and Syrian activists said Wednesday, highlights the far-reaching goals of Russia’s military involvement in Syria. It suggests that, for now, taking on Islamic State extremists in eastern Syria seems a secondary priority to propping up President Bashar Assad.
The development is almost certain to increase pressure on Western-backed rebels, who are battling multiple foes, and push more civilians out of the areas of fighting, potentially creating a fresh wave of refugees.
Russia began its air campaign Sept. 30, and Syrian troops and allied militiamen launched a ground offensive against rebels in central Syria a week later. Russia says its airstrikes are meant to weaken the Islamic State group and other ‘‘terrorists’’ in Syria, but Western officials and Syrian rebels say most of the strikes have focused on central and northern Syria, where the extremist group does not have a strong presence.
The official, who has deep knowledge of operational details in Syria, said the Iranian Revolutionary Guards — currently numbering around 1,500 — began arriving about two weeks ago, after the Russian airstrikes began, and have accelerated recently. The Iranian-backed group Hezbollah has also sent a fresh wave of fighters to Syria, he said.
Iranian and Syrian officials have long acknowledged Iran has advisers and military experts in Syria, but denied there were any ground troops. Wednesday’s statements were the first confirmation of Iranian fighters taking part in combat operations in Syria.
The main goal is to secure the strategic Hama-Aleppo highway and seize the key rebel-held town of Jisr al-Shughour in Idlib province, which Assad’s forces lost in April to insurgents that included Al Qaeda’s Nusra Front.
The loss of Jisr al-Shughour, followed by the fall of the entire province, was a resounding defeat for Assad, opening the way for rebels to threaten his Alawite heartland in the coastal province of Latakia. The official suggested the Syrian army’s alarmingly tenuous position around that time is what persuaded the Russians to join the fray and begin airstrikes two weeks ago.
The Syrian government and Iran had been asking Russia to intervene for a year, the official said, speaking on condition of anonymity because he was not authorized to discuss military affairs. He said the Russian ‘‘tsunami wave’’ has given allies such as Iran the cover to operate more freely in Syria.
His account of Iranian troops arriving ties in with reports from Syrian opposition activists, who reported a troop buildup in the northern provinces of Idlib and Aleppo. The Syrian Observatory for Human Rights also reported Wednesday that Iranian troops were arriving and being transported to a military base in the coastal town of Latakia, in the town of Jableh outside the provincial capital.
At least two senior Iranian commanders were killed in Syria in recent days, including General Hossein Hamedani, a senior Revolutionary Guard commander, who died Oct. 8 near Aleppo.
‘‘Syria will witness big victories in coming days,’’ said General Mohammad Ali Jafari, chief of Iran’s Revolutionary Guard, speaking Monday at Hamedani’s funeral.
‘‘Sending more troops from Hezbollah and Iran only increases the shelf life of the Syrian regime, which is destined to end,’’ said Major Jamil Saleh, the leader of Tajammu Alezzah, a CIA-backed Free Syrian Army faction. ‘‘It will only add more destruction and displacement.’’