Iranian-backed militias from Iraq have entered Syria to assist the government's counteroffensive against rebels who have taken control of Aleppo, the largest city.

According to the Syrian Observatory for Human Rights, based in Britain, approximately 200 Iraqi militiamen traveling in pickups crossed into Syria overnight.

"These are fresh reinforcements being sent to aid our comrades on the front lines in the north," a senior military official told the Reuters news agency. 

Fighting continues around Aleppo and Idlib

Led by Islamist group Hayat Tahrir al-Sham, insurgents launched a two-pronged attack on the city last week and moved into the countryside around Idlib and the neighboring Hama province.

Syrian and Russian airstrikes on rebel positions continued mostly in the two provinces.

Civil defense organization the White Helmets said at least 25 people were killed in airstrikes carried out by Syria and Russia in the region.

A spokesperson for the Russian government said "we of course continue to support Bashar Assad and we continue contacts at the appropriate levels.".

US, UN, EU urge de-escalation

The US, EU and UN on Monday all appealed for de-escalation, with UN Secretary General Antonio Guterres saying he was "alarmed" by the violence. They also appealed for a political process seeking to end the country's internal conflict.

"We want to see all countries use their influence — use their leverage — to push for de-escalation, protection of civilians and ultimately, a political process forward," US State Department spokesman Matthew Miller told reporters. 

In a statement from foreign affairs spokesman Anouar El Anouni, the EU also called for de-escalation and the protection of civilians, as well as criticizing Russia for participating in airstrikes over densely-populated areas.

The UN estimates that since the fighting flared up again in the long-running domestic conflict, between November 26 and December 1, at least 44 people have died and around 48,500 have been displaced in the area.

"Syrians have endured the conflict for nearly 14 years. They deserve a political horizon that will deliver a peaceful future, not more bloodshed," Guterres' spokesman Stephane Dujarric told reporters.

Iran reiterates support for Syria

On Monday, Iranian Foreign Ministry spokesperson Esmaeil Baqaei said advisors from Tehran would remain in Syria "in accordance with the wishes" of Damascus.

"We entered Syria many years ago at the official invitation of the Syrian government, when the Syrian people faced the threat of terrorism," he said.

Iranian Foreign Minister Abbas Araghchi met with Syrian President Bashar Assad in Damascus Sunday and announced Tehran's full support for his government.

"I clearly announce full-fledged support to President Assad, government, army and people of Syria by the Islamic Republic of Iran," Aragchchi said.

The Iranian foreign minister later arrived in Turkey, one of the rebels' main backers, where he said that both countries agree that Syria "must not become a center for terrorist groups."

His Turkish counterpart Hakan Fidan added that it would be a mistake to label the cause for the current escalation in Syria as "foreign interference."

Syrian Kurds were fleeing the fighting in large numbers after Turkish-backed rebels seized tal Rifaat from US-backed Kurdish authorities.

ftm/zc (Reuters, AP)