Britain, Germany and France called on Monday for an end to the military escalation in northwestern Syria, saying they were gravely concerned at recent violence which had led to the death of more than 120 civilians.
Last week, Syria's army, backed by Russian air power, launched ground operations against the southern flank of a rebel zone consisting of Idlib and parts of adjacent provinces.
"This military escalation must stop," the joint statement from the three countries, issued by Britain's Foreign Office, said.
"Airstrikes on population centers, indiscriminate bombardment and use of barrel bombs as well as the targeting of civilian and humanitarian infrastructures, notably schools and health facilities, are blatant violations of International Humanitarian Law."
Syrian troops on Monday captured five small villages and a hill on the edge of the northwestern province of Idlib, the last major rebel stronghold in the country, as the leader of the main insurgent group in the region called on every able person to "perform his religious duty" and join the fight.
Abu Mohammed al-Golani, the leader of the al-Qaida-linked Hayat Tahrir al-Sham, or HTS, claimed in a video interview with a local activist that government forces have suffered many losses while trying to advance into rebel-held areas over the past few days. Dressed in military uniform and holding an automatic rifle while seated under a tree, he claimed his militant group was gaining more fighters each day.
Al-Golani also said insurgents have the right to strike at a Russian base in Syria because "more than 90% of the Russian airstrikes are targeting women and children."
Syrian troops have been advancing on the rebels for nearly two weeks under the cover of Syrian and Russian airstrikes. Insurgents have also been blamed for firing rockets into Russia's air base in the nearby coastal province of Latakia.
Russia joined Syria's war in 2015, tipping the balance of power in favor of President Bashar Assad's forces. Opposition activists say Russian warplanes have been bombarding rebel-held areas intensely in recent weeks.
Much of Idlib is controlled by HTS, the largest and most powerful group in the area. Most of its fighters belonged to al-Qaida's affiliate in Syria, formerly known as the Nusra Front.
The capture of more territory by Syrian troops comes as part of the latest push by government forces against the rebel-held enclave. So far, dozens have been killed and more than 150,000 have been displaced. Idlib is home to some 3 million people, many of them internally displaced from other parts of the country.
The pro-government Syrian Central Military Media said government forces captured the villages of Hawash, Jabrieh, Tawbeh, Sheikh Idris, Jub Suleiman and Hawash hill on the southern edges of Idlib.
The villages are near the strategic village of Kafr Nabudah, which Syrian troops entered last week. Opposition activists reported government airstrikes and shelling of different areas in Idlib.
Syrian state media reported that insurgents fired rockets into the Christian government-held town of Suqailabiyah, killing one person and wounding five. The shelling came a day after a rocket attack on the town killed five children and a woman.
