BEIRUT—Shiite fighters, largely from outside Syria, are advancing toward the country's largest city backed by Russian airstrikes, giving the Assad regime the most significant momentum it has had in years just days ahead of new cease-fire talks.

Syrian and Russian airstrikes routed rebels from long-held territories in northern Syria, while pro-government forces led by Iranian, Lebanese Hezbollah, and other Shiite fighters are advancing on Aleppo, according to rebels on the ground and officials tracking the conflict.

They seized the village of Kifeen north of Aleppo on Sunday night, rebels said, putting the regime forces about 15 miles from the Turkish border—the closest they have been since late 2013.

More than 35,000 Syrians seeking to escape the assault over the past week have already converged on the border with Turkey, creating a new humanitarian crisis. Some are sleeping out in the cold in open fields or amid olive trees. Others have taken refuge in schools and mosques, waiting to be allowed into Turkey.

The offensive by President Bashar al-Assad's regime has turned the tide against rebels in the north and especially around Aleppo, Syria's largest city, which has been partly controlled by rebels since 2012. That effort may take months. But the gains offer the clearest example yet of the impact of Russia's foray into the Syrian war—something that rebels are unlikely to reverse without an infusion of military support from their regional and Western backers.

Meeting in Washington on Monday, Secretary of State John Kerry and Saudi Foreign Minister Adel al-Jubeir discussed a Saudi proposal to send ground forces to support the rebels. U.S. officials were welcoming but said they needed to know more.

Escalated Russian bombing coincided with United Nations-mediated peace talks on Syria held in Geneva last week, helping pro-government militias break a rebel siege on Shiite-majority villages and advance toward Aleppo city.

The talks quickly collapsed, with the U.S. and rebel groups laying much of the blame on Moscow and the Assad regime.

Moscow has ignored calls from the U.N. and U.S. to end its campaign—jeopardizing chances for much success when global powers meet Thursday in Munich to discuss implementing a cease-fire.

The landscape has quickly grown more tangled. Kurdish fighters who aligned with the U.S. in fighting Islamic State are now using the Russian airstrikes to make their own territorial gains at the expense of the weakened rebels.

German Chancellor Angela Merkel, in the Turkish capital Ankara to discuss the refugee situation, sharply criticized the bombing in Syria, which she said was "mainly from the Russian side."

"Over the past days, we have been not only appalled, but also horrified by the human suffering created for tens of thousands of people by airstrikes," she said.

World leaders have called on Turkey to open its border to the newly displaced wave. Turkish officials are reluctant to do so as the country is already hosting some 2.5 million Syrian refugees.

Rebels, cut off from their main northern supply route last week, said they feared the campaign would next expand westward to close their last line into Aleppo, which runs from the western Bab al-Hawa crossing at Turkey's border.

"The military scales have entirely flipped," said Mustafa Amin, a political leader with the Levant Front, one of the biggest non-jihadist rebel alliances in Aleppo.

Mr. Amin, whose group has received American TOW antitank missiles, said the rebels' focus was no longer on controlling territory but "to continue the battle as we much as we can, and to chip away at the regime's allies."

"There is no regime advance," he added. "In the air, it is Russian planes. And on the ground, there are Afghan, Hezbollah, and Iranian fighters. And in the operations room, there are Iranians."

Iran has recruited thousands of Afghans, many of them refugees living in Iran, to fight on the government's side in Syria. A Human Rights Watch report in January said the Afghan fighters are organized and commanded by Iranian military officials and have fought at every major battlefield in Syria, including around Damascus and Aleppo.

In addition, Chechen leader Ramzan Kadyrov said on Russian state television late Sunday that special forces from his mostly Muslim region are on the ground in Syria to assist the Kremlin's air campaign.

A Western diplomat tracking the war said the Russian intervention has "absolutely created a new reality on the ground that will be difficult to reverse."

Moscow began bombing Syria in late September, vowing to target Islamic State. U.S. and other Western officials say the strikes have focused on anti-Assad rebels, including civilians in rebel-held areas. Russia denies targeting civilians.

The battle for Aleppo, which had a prewar population of more than two million, once focused on the rebel potential to topple a major urban center. But now, much like the broader battlefield in Syria, it is no longer a viable fight between anti-Assad forces and the regime, rebels and analysts said.

As Russia aids the regime, the U.S.-led coalition in Syria has been focused on fighting Islamic State and sealing its access to the Turkish border, rather than bolstering rebels, they said.

In addition to the Russian bombing, the regime's recent gains were enabled by a new flood of Shiite pro-government fighters from Iran, Iraq and Lebanon, rebels said.

Further squeezing them are opportunistic moves by U.S.-backed Syrian Kurdish militias, who are seeking to consolidate their own territory in northern Syria at the expense of other rebel groups.

The Kurdish fighters, known as the YPG, have benefited from Russian airstrikes and in some cases have received direct Russian military aid, Kurdish fighters said, showing how messy alliances in the area have become.

Advances in recent weeks by the Kurdish-controlled Syrian Democratic Forces—an alliance that also includes non-Kurdish rebels—northwest of Aleppo had already weakened rebels and cut off some of their supply lines when pro-regime forces escalated their offensive there last week, Kurdish fighters and rebels said.

Syrian Democratic Forces spokesman Talal Silo said in an interview at his headquarters in Hasakah, in northern Syria, that rebels were given a choice to join his alliance. "They will be finished by the regime," he predicted.

Kurds have to take territory from both Islamic State and the rebels fighting Islamic State to link up their territory in northern Syria.

On Monday, the YPG took control of two villages from rebels north of Aleppo, activists said, after seizing over the weekend another village on the outskirts of the Minnigh air base, a rebel-held air base now surrounded from the north and south by Kurdish fighters.

A United Nations report on Monday accused the Syrian government of "extermination as a crime against humanity" in jails across the country. The U.N. commission of inquiry, which based its finding on 621 interviews, said thousands of detainees have been killed while held by different sides in Syria's conflict. But it went on to say it was apparent that government authorities administering prisons and detention centers were aware that deaths on a massive scale were occurring.

Damascus has previously denied it systematically tortures or kills people in detention.

Felicia Schwartz in Washington and Dana Ballout contributed to this article.

Write to Nour Malas at nour.malas@wsj.com and Raja Abdulrahim at raja.abdulrahim@wsj.com

(END) Dow Jones Newswires

February 09, 2016 01:45 ET (06:45 GMT)