Islamic State fighters have attacked the Syrian city of Kobane, months after being driven out in a symbolic battle that made international headlines.
They detonated car bombs and launched an assault. Kurdish media say at least 50 civilians have been killed, including 20 in a nearby village.
IS has recently suffered a string of defeats to Kurdish forces.
But in another attack on Thursday, it seized parts of the key north-eastern city of Hassakeh.
The apparent two-pronged IS offensive came as Kurdish fighters from the Popular Protection Units (YPG) cut a major supply line for IS near Raqqa.
Raqqa is the de facto capital of the caliphate whose creation IS announced a year ago after it captured large swathes of northern and western Iraq.
Analysis: Quentin Sommerville, BBC Middle East correspondent:
Kobane still matters to IS. It was never important strategically, but this latest attack shows that its loss, after five months of heavy street-to-street fighting and coalition aerial bombardment, still hurts IS.
As was the case last November when a huge vehicle bomb exploded at the same spot, questions are being asked if the attackers made it in from the Turkish side, and if so, why Turkey didn't stop them.
Thursday's assault is a reminder, too, that IS, despite recent losses in the area, is still very much active and capable of offensives. Overnight they also attacked Hassakeh to the east, a far bigger prize.
Despite the narrative of the last few weeks, IS is far from being on the back foot.
The IS attack on Kobane began with militants detonating a car bomb, followed by an assault from dozens of fighters from a number of directions.
There are reports that some may have hidden themselves among returning refugees, and disguised themselves by wearing Kurdish militia uniforms.
A reporter with the privately-owned Kurdish television station NRT said they infiltrated the city at dawn "and then went on a killing spree", targeting people in their homes as they slept.
Hours later two more car bombs exploded.
In the nearby Kurdish village of Bakha Botan, at least 20 civilians - among them women and children - were reportedly shot dead by IS fighters.
At least eight IS militants and a number of Kurdish fighters have been killed in the fighting in Kobane, activists say.
One senior local official told BBC Persian that the situation in Kobane itself was now "under control" but clashes were continuing on its eastern and northern outskirts.
Speculation that the militants had entered Kobane from Turkey were strongly rejected by Turkish officials.
Hassakeh, about 270km (180 miles) east of Kobane, has been under the control of both government forces and Kurdish fighters, and IS militants have been trying to capture it for months.
The militants reportedly struck at government-held neighbourhoods and captured two districts. Syrian state TV said local people were being forced from their homes.
At least 30 government loyalists and 20 militants are believed to have been killed, along with an unknown number of civilians.
Hassakeh is the largest city in north-eastern Syria, with an estimated 500,000 residents, in addition to thousands who have fled there to escape violence elsewhere.
Following IS's defeat in Kobane in January, as many as 35,000 of the city's 400,000 population had begun to return.
More than 200,000 Syrians have lost their lives in four years of armed conflict, and more than 11 million others have been forced from their homes.
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