The group seized the town of Akhtarin, 50km (30 miles) northeast of Aleppo city, where Syrian rebels are holed up.
The BBC's Rami Ruhayem says that if confirmed, it would be a significant expansion for IS fighters.
The group holds large swathes of Syria and Iraq, declaring the creation of a caliphate, or Islamic state.
The UK-based Syrian Observatory for Human Rights (SOHR) - an activist group that monitors the conflict - says IS took control overnight of Akhtarin and Turkmanbareh, another town close to the border with Turkey.
The militants also took a string of nearby villages from Syrian opposition groups who are fighting President Bashar al-Assad. They include Masoudiyeh, Dabiq and Ghouz, according to the SOHR.
Correspondents say the seizing of the towns and villages could threaten Syrian rebel supply lines into the city of Aleppo.
Opposition groups in Aleppo have been fighting President Assad for over three years, but are being squeezed by IS gains in the north and a government offensive from the south.
Diplomats say Syrian government forces have only recently begun taking on IS after the group seized more territory in the country and across the border in Iraq.
IS has had control of several parts of Syria since 2013 and used the northern town of Raqqa as its hub in Syria after capturing an army base there last month.
Syria's conflict began in March 2011 as a popular uprising against President Assad's regime, but has since turned into a brutal civil war that has killed at least 170,000 people.
Some nine million people, or a third of Syria's pre-war population, have fled their homes.
Islamic State (IS) militants in Syria have taken control of several towns in the northern province of Aleppo, according to reports from activists.