The Islamic State group launched a two-pronged offensive in northern Syria on Thursday after several setbacks, re-entering the symbolic battleground town of Kobane and seizing parts of the city of Hasakeh.
In southern Syria, an alliance of rebel groups, including Al-Qaeda affiliate Al-Nusra Front, also attacked government-held areas of the city of Daraa.
Analysts said the surprise IS assaults were aimed at diverting Kurdish forces after they scored a series of victories and advanced on the jihadists' Syrian stronghold of Raqa.
Kobane, on the border with Turkey, became an important symbol in the battle against IS after the group launched a bid to take it last year.
Kurdish forces backed by US-led air strikes waged a four-month battle to repel the group, finally securing the town in January.
But on Thursday, the jihadists returned, detonating a suicide car bomb near the border crossing adjacent to Kobane as they launched an assault.
"Fierce clashes erupted afterwards in the centre of the town and there are bodies lying in the streets," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights monitor.
He said at least 12 civilians and Kurdish fighters had been killed in the car bomb and the subsequent fighting in the town, along with eight IS militants.
A few hours later, two more car bombs detonated near the border, but there were no immediate details on casualties.
IS forces also entered a Kurdish village some 20 kilometres (12 miles) south of Kobane on Thursday morning, executing at least 23 residents, among them women and children, the Observatory said.
The jihadists withdrew from Barkh Butan after US-led coalition strikes on the outskirts of the village and the arrival of Kurdish forces, the monitor said.
The IS assault on Kobane prompted angry Kurdish accusations that Turkey had allowed the jihadists to enter Syria from its territory, a claim Turkish officials dismissed as "baseless".
Ankara says Syria's Kurdish forces are linked to the Turkish Kurdistan Workers' Party (PKK), which it lists as a "terrorist group," and has eyed their advances against IS with suspicion.
Since being pushed out of Kobane at the start of the year, IS has suffered a string of defeats at the hands of the Kurdish People's Protection Units (YPG) and their Arab rebel allies.
The YPG seized the border town of Tal Abyad to the east on June 16 and then drove on south towards Raqa, IS's de facto Syrian capital.
Tal Abyad had been a key conduit for IS, allowing it to transport weapons and fighters to and from Raqa.
As the Kobane attack began on Thursday, IS forces also entered the northeastern city of Hasakeh.
Charles Lister, a visiting fellow at the Brookings Doha Centre think tank, said the two-pronged assault was a diversionary tactic after Kurdish forces advanced to within 55 kilometres (35 miles) of Raqa this week.
"Events overnight in Kobane and Hasakeh have displayed classic IS strategy, whereby unexpected, spectacular attacks have been launched as diversionary operations aimed at distracting the Kurds from their role approaching Raqa," said Lister, author of "The Syrian Jihad," a book on IS.
By Thursday morning, IS fighters controlled two neighbourhoods of Hasakeh, capital of a Kurdish-majority province in the northeast, the Observatory said.
At least 30 government loyalists and 20 jihadists were killed in the fighting.
Control of Hasakeh is divided between government loyalists and Kurdish militia who are mostly present in the city's north and northwest.
IS has sought repeatedly to enter the city, including earlier this month when it advanced to the southern outskirts before government forces pushed it back.
State television acknowledged "heavy clashes ongoing between Syrian army troops and National Defence Forces against IS terrorists in the Al-Nashwa district of Hasakeh."
On Twitter, IS reported the assault, saying "in a surprise attack facilitated by God, the soldiers of the caliphate took control of Al-Nashwa district and the areas around it."
In southern Syria, government troops came under attack in Daraa, another provincial capital.
An alliance of rebel groups including Al-Nusra attacked government-held parts of the city, the Observatory said.
A military source in Damascus told AFP "the fighting between the two sides in Daraa city that began overnight is continuing," and state television said at least six people had been killed in rebel mortar and makeshift rocket fire on the city.
More than 230,000 people have been killed in Syria since the conflict began with anti-government demonstrations in March 2011.