Islamic State of Iraq and the Levant (ISIL) fighters have launched attacks on two fronts in northern Syria, re-entering the Kurdish town of Kobane and seizing parts of the city of Hasakah.
Dozens of ISIL fighters attacked Kobane on the border with Turkey, where at least 12 people were killed in a car bomb attack at the start of the offensive on Thursday morning.
ISIL fighters were wearing Kurdish and Free Syrian Army uniforms, the sources told Al Jazeera, as they attacked from three sides and took several positions inside the battleground town.
Al Jazeera's Nisreen El Shamayleh, reporting from Amman, said several ISIL fighters "carried out suicide attacks; decimated themselves and caused a lot of casualties" after entering the city.
"There's a lot of fighting going on there, that we understand is ongoing," our correspondent said.
"Dozens of people have been trying to flee."
The Kurdish group YPG asked civilians to stay home as it sent reinforcements to the town.
Kobane explained: What's so special about it?
The fighting prompted Kurdish activists and Syrian state television to accuse Turkey of allowing ISIL to attack Kobane from its side of the border.
A Turkish foreign ministry spokesman later "strongly denied" that the ISIL fighters crossed into Syria from Turkey.
Kurdish forces in January had reclaimed Kobane from ISIL in a victory touted by Anwar Muslim, the prime minister of the self-declared Kurdish canton of Kobane, as "the beginning of the end for Daesh [ISIL]".
Losing Kobane after more than four months of intense fighting was seen as a significant propaganda blow to ISIL after it had invested extensive military resources to capture the isolated border town.
"Daesh [ISIL] took most of the places it wanted in Syria and Iraq but could not capture Kobane," Muslim told Al Jazeera at the time.
|Al Jazeera [Daylife]
ISIL storms Hasakah
Meanwhile, ISIL launched an overnight offensive on the largely Kurdish city of Hasakah in northeast Syria where dozens of Syrian and ISIL fighters were reportedly killed, sources told Al Jazeera.
The UK-based Syrian Observatory for Human Rights group estimated that at least 30 Syrian soldiers and 20 ISIL fighters died in the raid.
Dozens of people fled Hasakah towards the northern countryside after the sudden offensive, Al Jazeera's sources reported.
Fighting was ongoing on Thursday morning as ISIL stormed the city from its southern entrance in its attempt to take control of more territories in Hasakah.
A suicide bomber also blew up a car bomb at the city's western entrance.
Fighting in Aleppo and Deraa
Meanwhile, after two years of fighting for Layramoun Square in Aleppo, rebels were saying on Thursday that they had seized the area from government forces.
They also took control of a surrounding government barracks northwest of the city, Al Jazeera's sources said.
Syrian rebels and groups including the al-Qaeda-linked Nusra Front also attacked government-held areas of the southern city of Deraa overnight.
Rebels previously held Deraa's eastern half while the Syrian government held western areas of the city.
Heavy fighting in Deraa is continuing, according to Al Jazeera's sources.
ISIL prisoners beheaded
In a separate development, ISIL beheaded 12 men from rival Syrian rebel movements accused of fighting against them, in a video released on Thursday.
It is the latest in a long series of mass beheadings by ISIL, and comes two days after the group released a video showing it killing 16 people in neighbouring Iraq, drowning some of them in a cage.
Four men were killed with a rocket-propelled grenade fired at a car and seven by wrapping explosive cord around their necks and detonating it.
Three of those killed in the new video were from Jaysh al-Islam, one of the main rebel groups in the Damascus area, and a fourth from al-Nusra Front, al-Qaeda's Syrian affiliate and ISIL's main rival in the country.
Source: Al Jazeera