Syrian Kurdish fighters say they have taken control of 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 allows them to link up the other pockets they control along the Turkish border, from Iraq in the east to Kobane in the west.
"The whole city is under our control and there is no more fighting," Huseyin Kocher, a Kurdish YPG commander in Tal Abyad told the BBC.
"Our people should know that we are going to clean all the remnants of IS in northern Syria."
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 away, a YPG commander said.
A spokesman for the Syrian rebel group Burkan al-Furat, which is fighting alongside the YPG, said there had been "intense clashes" in eastern and southern Tal Abyad.
More than 16,000 civilians flee fighting
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 3000 refugees arrived at the crossing on Monday, Turkish state television reported.
This is not the first time Syrian Kurdish forces have dealt a blow to Islamic State. But if the Kurdish YPG manages to hold on to Tal Abyad, it will be the most serious one.
The border town, which lies 100 kilometres north of Raqqa, the self-declared capital of the Islamic State, is one of the most important border gates for the extremist group.
Syrian Kurdish forces have already shown they can be a match for IS during their resistance in the nearby border town of Kobane.
But the coalition air strikes, which are co-ordinated with the Kurds, proved to be a strong factor in helping the Kurds to succeed.
Kurdish forces have built up their campaign against the group by recapturing the strategic Abdulaziz mountains to the southeast of Tal Abyad and enlisting support from Arab tribes and FSA factions, including the Euphrates Volcano, alongside Assyrian Christian militia.
Forces belonging to IS are stretched and are fighting several groups of regional forces deep in Iraq and Syria.
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".
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.