Syrian Kurdish forces, backed by U.S. air strikes, dealt a strategic blow on June 15 to the Islamic State, seizing a key border town that was one of the militants' main supply routes from Turkey.
The capture of Tel Abyad by the Kurdish YPG and smaller Syrian rebel groups means the Syrian Kurds effectively control some 400 kilometers of the Syrian-Turkish border that had been a critical conduit for foreign fighters joining Islamic State.
Tel Abyad also has been a key transit point for militants to smuggle weapons and oil.
YPG claimed on its website and in interviews with reporters late on June 15 that it was in full control of the town.
"The aim of the fighters is to cut the supply route of Islamic State, and by taking Tel Abyad we will be able to cut the supply route, and we have already closed in on them," Syrian Kurdish official Idriss Nassan told the German news agency dpa.
Video recorded from the Turkish side of the border showed fighters waving the flag of the YPG near the spot where, only 24 hours earlier, Islamic State had pushed fleeing civilians back from the border fence, dpa reported.
The green, white, and black tricolor adopted by Syrian rebels was seen draped over the border crossing itself in the footage published by the Turkish DHA news agency.
The Syrian Observatory for Human Rights, a group based in the United Kingdom that monitors the war, said Islamic State fighters are resisting in some pockets, though the well-organized YPG was in control of most of the town.
The YPG has proven the most effective force combating the Islamic State in Syria, and because of that has received massive air support from the United States and its allies since the battle for Kobani.
The YPG now controls more than half of the 899-kilometer border between the two countries, from Syria's far northeastern corner to just east of the town of Jarabulus.
With Tel Abyad now out of reach, the militants' only route between the border and the bulk of their territories in Syria and neighboring Iraq will be a longer one via Aleppo Province further west.
The YPG-led forces also seized control of the road linking Tel Abyad to the Islamic State's de facto capital of Raqqa city, about 80 kilometers to the south, cutting off a supply route which militants had used to send reinforcements.
The Syrian observatory said at least 40 militants were killed in U.S.-led air strikes as they fled southward.
The YPG-led forces advanced into Raqqa after making big gains against Islamic State in neighoring Hasaka Province since early May, also with the help of the U.S.-led alliance.
While Islamic State was being driven back in Hasaka, it was advancing elsewhere in Syria against government forces, notably in Palmyra, which the jihadists seized from the government in mid-May.
While the advance has brought YPG deep into the militant stronghold of Raqqa Province, its gains have concerned Turkey, which is worried the expansion of Kurdish power will inflame separatist sentiment among its own Kurdish minority.
Turkish President Tayyip Erdogan has voiced concern about the latest YPG-led offensive, saying Kurds were taking over areas from which Arabs and Turkomans were being displaced.
The fighting near the border has forced more than 18,000 people to cross into Turkey from Syria, aid workers say.
Erdogan has accused the West of backing Kurdish "terrorists." He has also said the outcome could eventually threaten Turkey's border.
Turkey views the YPG as part of the PKK, which has fought a decadeslong insurgency against Ankara and is considered a terrorist organization by Turkey, the United States, and the European Union.
The YPG counters that it protects citizens of all religions and ethnicities in areas it controls, with the sympathetic Hawar news agency frequently publishing clips of Arab villagers expressing gratitude to YPG forces.