Turkish-backed Syrian rebels are advancing on Dabiq, a symbolic stronghold of so-called Islamic State.
The small town holds great value to IS because of a prophecy of an apocalyptic battle, and features heavily in its propaganda.
The operation began as US and Russian envoys met in Switzerland to discuss possible routes to a new ceasefire.
No statement was issued after the talks although both the US and Russian envoys spoke of “ideas” emerging.
Since a brief truce collapsed last month, Syrian government forces backed by Russian air strikes have intensified their bombardment of rebel-held areas in Aleppo.
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Aid agencies say a 72-hour ceasefire is urgently needed to allow supplies in and civilians out of devastated areas in the east of the city.
Apocalyptic prophecy
Turkish President Tayyip Erdogan confirmed fighters were moving on Dabiq, which lies about 10km (6 miles) from the Turkish border.
Rebel fighters backed by Turkish airstrikes have been edging closer to the town for days, seizing villages around it and all but isolating it.
A bombardment was taking place as part of the offensive on Saturday, a monitoring group and a rebel commander said.
Dabiq is important to IS because it is named in Islamic apocalyptic prophecies as the site of an end-of-times showdown between Muslims and their enemies. The militant group named its magazine after the town.
The advance on Dabiq is part of a wider offensive launched by an alliance of Syrian rebel groups, supported by Turkish forces, in late August.
They are trying to drive IS militants and Syrian Kurdish fighters from an area along Turkey’s border with Syria. Since it began, they have captured the key towns of Jarablus and al-Rai.
‘Different approaches’
Meanwhile, US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov met delegates from Turkey, Saudi Arabia, Iran, Egypt, Iraq, Jordan and Qatar in Lausanne to discuss ways to broker a new ceasefire.
Both sides had played down expectations of any breakthrough.
Mr Kerry and Mr Lavrov met privately for 40 minutes ahead of the wider talks, which lasted more than four hours.
The group made no statement after the meeting, but Mr Lavrov told Russian media that the participants agreed to continue contacts.
Both envoys spoke of “ideas” emerging, and Mr Kerry said it might be possible to shape “some different approaches” from the meeting.
“There were some difficult moments, where there was obviously tension, but everybody was constructive,” he said.
Image copyright Reuters
Image caption Rebel-held areas of Aleppo have faced daily air strikes
‘Little agreement’: By Imogen Foulkes, BBC News, Lausanne
Expectations for these talks were never high, but their break up after less than five hours is a further sign of how little agreement there is over how to bring peace to Syria.
The US has accused Russia of war crimes because of its support for the bombing of Aleppo. Moscow has suggested Washington is not really trying to isolate Islamist extremists from more moderate opposition groups.
Regional powers like Iran are fighting side by side with Syrian troops. Saudi Arabia and Qatar are arming the opposition.
These countries wield huge influence in Syria, but their talks brought no apparent agreement and no public statement. When pushed, as the motorcades headed to the airport, one western diplomat simply described them as businesslike and courteous.
Syria’s President Bashar al-Assad has vowed to “keep cleaning” Aleppo of rebels and told a Russian newspaper that winning in the city would be a “springboard” to winning in the rest of the country.
Organisations including Save the Children, Oxfam, the Norwegian Refugee Council and the International Rescue Committee have issued a plea “to establish a ceasefire of at least 72 hours in east Aleppo” to allow evacuations and delivery of aid.
Image copyright Reuters
Image caption Aid groups say a truce is needed in Aleppo to allow supplies in and let civilians out
There are now no safe areas left in rebel-held parts of the city, according to REACH, an organisation that contacts people there regularly in order to gather humanitarian reports.
About 275,000 people live in the besieged areas, and aid organisations have not been able to get to them since the siege resumed on 4 September.
More than 370 people, including nearly 70 children, have been killed in the bombardment of eastern Aleppo, according to the UK-based Syrian Observatory for Human Rights.
The monitor said dozens of civilians including children have also died in rebel bombardment of western Aleppo, which is controlled by the Syrian government.
A war that started with an uprising against President al-Assad has now split Syria into many parts. It has been going on for over five years and it has claimed 300,000 lives.
