By Lisa Barrington and Suleiman Al-Khalidi

BEIRUT/AMMAN (Reuters) - U.S.-backed Syrian forces edged closer to an Islamic State stronghold on the border with Turkey on Saturday while Russia's defense minister visited President Bashar al Assad to discuss military operations.

The visit came only hours after the Syrian army and its Iranian-backed militias, which have been supported by Russian air power, lost several villages to Islamist rebels as they made significant advances in the countryside south of Aleppo.

The UK-based Syrian Observatory for Human Rights said the rebel capture of three villages from government control - Zeitan, Khalsa and Barna - had caused significant losses among government forces and their allies.

The villages lie in an strategic area near a main highway that links Aleppo with the capital Damascus. Government forces captured the area at the end of last year in a major offensive, assisted by Iranian-backed militias and Russian jets.

State media said Russian Defense Minister Sergei Shoigu visited an airbase in the coastal Latakia province on Saturday after his talks with Assad in Damascus.

Russia's military intervention in Syria in September helped turn the tide in Assad's favor after months of gains in western Syria by rebel fighters, who were aided by foreign military supplies, including U.S.-made anti-tank missiles.

Russia, which has been bombing opposition-held areas, is blamed by the opposition and rights activists for causing hundreds of civilian deaths and targeting hospitals, schools and infrastructure in what they say are indiscriminate attacks.

An escalation in Russian and Syrian air and artillery strikes in recent weeks around a highway to rebel held parts of the northern city of Aleppo has made the road virtually impassable, putting hundreds of thousands of people under siege and worsening their humanitarian plight.

A 48-hour ceasefire in Aleppo announced by Russia on Thursday has had little impact on fighting, and air strikes and shelling have continued in and around the city.

At least seven people died in rebel shelling of a neighborhood of Aleppo held by the Kurdish YPG militia at dawn on Saturday, the Observatory said.

Syrian helicopters also threw barrel bombs on several residential areas in opposition-held quarters.

CLOSE TO MANBIJ

Hundreds of people have been killed in Aleppo since peace talks broke off in April, as President Assad seeks to regain control of what was Syria's largest pre-war city, now split between rebel and government sectors.

Washington and some other Western countries that have called on Assad to step down accuse Russia of focusing mostly on strikes against the moderate so-called Free Syrian Army (FSA), and less on attacking hardline Islamic State militants.

On the border with Turkey, U.S.-backed Syrian forces fought to the western entrance of Manbij city for the first time since a major offensive to seize the last territory held by Islamic State militants on the frontier, a source in the Syrian group said on Saturday.

The source from the Syria Democratic Forces (SDF) alliance - which includes the powerful Kurdish YPG militia, Arab fighters and is being helped by scores of U.S. special forces - told Reuters its troops were now almost two km from the city center.

Since the start of the offensive on May 31, the SDF has taken dozens of villages and farms around Manbij but has held back from entering the city with many thousands of people still trapped there. [L8N1961B1]

The SDF is one of a number of sides fighting in Syria's complex civil war which has pitted rebels against Assad and is now in its sixth year.

The Observatory, which tracks violence across Syria, confirmed heavy fighting broke out for the first time on the edge of the city, adding that the militants had rammed a suicide vehicle into an SDF outpost but gave no details.

Syrian government forces and some rebel groups are also fighting separate battles against Islamic State. The SDF has largely avoided fighting government forces and focuses on battling the hardline militants.

(Additional reporting by Rodi Said near Manbij in Syria and Polina Devitt in Moscow; editing by Gareth Jones and David Clarke)