The Islamic State group recovered after several setbacks on Thursday and launched a two-pronged military attack in northern Syria, re-entering the symbolically important 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, attacked government-held areas of the city of Daraa.
Kobane, on the border with Turkey, is an important symbol in the battle against the jihadists, as Kurdish militia, supported by US-led airstrikes, wrested control of the town from the jihadists in January after four months of bloody combat.
The jihadists reentered the town by using a suicide car bomb, the Syrian Observatory for Human Rights said.
"IS detonated a suicide bomb in the area near the border crossing with Turkey, killing at least five people," Observatory director Rami Abdel Rahman said.
"Fierce clashes erupted afterwards in the centre of the town and there are bodies lying in the streets," he added, without giving a specific number.
Local Kurdish official Idris Nassan confirmed that IS fighters had penetrated the town.
"Daesh or Daesh-related terrorist groups are trying to create confusion to avenge their defeat and force Kurds to flee," he told AFP, using the Arabic acronym for IS.
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 farther east on June 16 and then drove on south towards the Euphrates valley city of 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.
The Kurdish advance to within 55 kilometres (35 miles) of Raqa this week prompted the IS counteroffensive, said Charles Lister, a visiting fellow at the Brooking Doha Centre think tank and author of "The Syrian Jihad," a book on IS.
Overnight, IS fighters seized 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, which continued into Thursday morning. "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," Lister said.
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 the jihadists had penetrated the city.
"Heavy clashes ongoing between Syrian army troops and National Defence Forces against IS terrorists in the Al-Nashwa district of Hasakeh," it said in a breaking news alert.
IS media also reported the assault on the city.
"In a surprise attack facilitated by God, the soldiers of the caliphate took control of Al-Nashwa district and the areas around it," the group said in a statement posted on Twitter.
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.
The attack followed a series of advances by rebels in the region, much of which is under opposition control.
State media acknowledged the attack, saying "six people were killed and 13 wounded in a terrorist assault on Daraa city with mortar rounds and gas canister bombs."
It said army units had "foiled attempts by terrorist groups to attack several military positions in Daraa province," although it made no mention of fighting inside Daraa city.
More than 230,000 people have been killed in Syria since the conflict began with anti-government demonstrations in March 2011.