Syrian Kurdish fighters say they have encircled the Islamic State-held town of Tal Abyad, cutting off a major supply route for the jihadist group.
Reinforcements are being sent by the Kurdish Popular Protection Units (YPG) to help secure the main road south to Islamic State’s headquarters at Raqqa.
The fighting has prompted thousands of civilians to flee to nearby Turkey.
The YPG advance has been supported by Syrian rebel groups and air strikes by the US-led coalition against IS.
For the Kurds, capturing Tal Abyad would help them link up the other pockets they control along the Turkish border, from Iraq in the east to Kobane in the west.
YPG units advancing from the east and west reached the outskirts of Tal Abyad on Sunday, after days of fierce clashes during which they seized a string of villages.
On Monday afternoon, the units met south of Tal Abyad after taking the road to Raqqa, which is 80km (50 miles) away, a YPG commander said.
“Tal Abyad is completely surrounded,” Hussein Khojer told the AFP news agency.
“There is nowhere Daesh can escape to,” he added, using a pejorative term for the jihadist group based on the acronym of its former name in Arabic.
A spokesman for the Syrian rebel group Burkan al-Furat, which is fighting alongside the YPG, said there were “intense clashes” in eastern and southern Tal Abyad.
The Kurdish assault and US-led air strikes have prompted more than 16,000 civilians to flee their homes and cross the border with Turkey.
The refugees were stranded at the border fence over the weekend after Turkey closed the Akcakale crossing, saying it would only allow them to enter in the event of a humanitarian tragedy.
But on Sunday afternoon, dozens managed to cross through holes cut in the fence before being rounded up on the Turkish side of the border by Turkish soldiers.
Later, refugees were seen passing through Akcakale after local officials said they had been given permission to reopen the crossing by the government in Ankara.
Up to 3,000 refugees arrived at the crossing on Monday, Turkish state television reported.
IS ‘given a beating’
YPG fighters have been moving into IS territory on either side of Raqqa province since forcing the jihadists to withdraw from Kobane in January.
The US deputy special presidential envoy for the international coalition against IS, Brett McGurk, said on Sunday that the Kurds were “really giving a beating” to IS.
But on Monday, a group of 15 Syrian rebel factions accused the YPG of carrying out a “sectarian and ethnic cleansing campaign” against Arab and Turkmen civilians from Tal Abyad and the western countryside of neighbouring Hassakeh province as it advanced into IS territory.
YPG spokesman Redur Khalil strongly denied the allegation, calling the rebel factions “bankrupt”.
Turkey’s President Recep Tayyip Erdogan meanwhile said he was troubled by the Kurdish advance, claiming it might “lead to the creation of a structure that threatens our borders”.
The YPG is the armed wing of the Syrian Kurdish Democratic Unity Party (PYD), which is itself an offshoot of the banned Turkish Kurdistan Workers’ Party (PKK).
In a separate development on Monday, rebels shelled a government-held district in the second city of Aleppo, killing at least a dozen people and wounding more than 100 others, including many children, activists and state media reported.