Islamic State militants in Syria have stormed government-held neighbourhoods in the predominantly Kurdish north-eastern city of Hassakeh, capturing several areas.
The attack came after IS suffered several setbacks in northern Syria against Kurdish forces over the past weeks.
The city of Hassakeh is divided between Syrian president Bashar Assad's forces and Kurdish fighters.
IS also staged a new attack on the Kurdish town of Kobani, which resisted a months-long assault by the Islamic militants before driving them out in January.
An activist group said 10 people died in the fighting today - the first time in six months IS had managed to enter Kobani, which lies along the Syria-Turkey border.
In Hassakeh, Redur Khalil, a spokesman for the Kurdish People's Protection Units, or YPG, said IS militants attacked government-held neighbourhoods on the southern edge of the city, and captured some areas.
Syrian state TV reported intense clashes inside Hassakeh's southern neighbourhood of Nashawi. According to the report, IS fighters killed several people they captured in the city, including the head of a military housing institution. It said the militants sustained many casualties, including the commander of the group who is a foreign fighter.
IS tried to storm the city earlier this month and reached its southern outskirts before facing strong resistance from Syrian government troops who pushed them away.
The attacks on Hassakeh and Kobani came days after YPG fighters and their allies captured the IS stronghold of Tal Abyad on the border with Turkey and the town of Ein Issa to the south. Kurdish fighters have been advancing since January under the cover of air strikes by the US-led coalition.
But in neighbouring Iraq, government forces and allied Shia militiamen have been slow in retaking IS-held territory. The Iraqis have also suffered occasional losses.
Iraqi troops drove IS militants from Saddam Hussein's home town of Tikrit in April, but lost Ramadi, the capital of Anbar province west of Baghdad, last month.
In June last year, IS launched a blitz, capturing large parts of both Syria and Iraq and subsequently declared an Islamic caliphate on the territory it controls. A major IS attack was widely expected during the Muslim holy month of Ramadan, which began last week.
In an audio message on Tuesday, IS spokesman Abu Muhammad al-Adnani urged Sunni Muslims to use the time of piety and dawn-to-dusk fasting during Ramadan to wage jihad and seek martyrdom.
"Attack them everywhere and shake the ground beneath them," he said. It was not possible to verify the recording, but it resembled previous audio statements from the group.
Al-Adnani referred to the recent battlefield setbacks for IS, saying the faithful "may lose a battle or battles and may lose towns and areas, but will never be defeated".
Thursday's attack in Kobani involved at least one suicide car bombing, according to Kurdish officials and activists.
Ghalia Nehme, a commander with the Kurdish Women's Protection Units, said that "a group of fighters deployed in some areas of Kobani".
"We are defending a position now," she added.
Another Kurdish official in Kobani, Idriss Naasan, said the fighting was intense in the morning but became more sporadic by midday. He said the extremists appear to have infiltrated from villages south of Kobani.
"We hear cracks of gunfire every now and then," Naasan said. He added that some explosions could still be heard but that it was unclear what those were.
The Britain-based Syrian Observatory for Human Rights, which tracks Syria's civil war, said Thursday's fighting in Kobani had killed 10 civilians and Kurdish fighters, and also three IS extremists.
After months of bloody street fighting, the Kurdish forces in Kobani, which lies along the Syria-Turkey border, succeeded in pushing out IS militants earlier this year. That was a landmark victory against IS, enabled in part by the US-led coalition's air strikes.
Two Turkish officials said Thursday's attack in Kobani involved a suicide bomber who detonated his car near the border gate that separates Kobani from the Turkish town of Mursitpinar.
The first official, from the local governor's office, said that 41 wounded were taken across the border to a hospital in Turkey. Surveillance footage seen by the Associated Press showed a fiery explosion rocking Kobani in the dim light of dawn.
The official said the video came from one of the 24 cameras monitoring the border crossing.
The second official, who is with the district government, put the number of wounded at 43 and said that sporadic gunfire could still be heard from the other side of the border later in the morning as well. He said one person, a child, had been killed.
Syrian state TV said the extremists crossed from the Turkish side of the border into Kobani, adding that there are casualties. It gave no further details.