Fighters from the militant Islamist al-Nusra Front have captured a Syrian oil town near the city of Hasaka, foreign-based anti-Assad organisations say.Video of gunmen celebrating at a battle-scarred compound said to be in al-Shaddada was posted on social media.
Thirty al-Nusra fighters, 100 Syrian soldiers and "dozens" of civilians were killed, the UK-based Syrian Observatory for Human Rights reports.
Forces opposed to President Bashar al-Assad made other advances this week.
An air base was captured along with aircraft near the northern city of Aleppo and the country's biggest hydro-electric dam, Thawra, was overrun in the north-east.
Nearly 70,000 people have been killed on all sides since the uprising against President Bashar al-Assad began nearly two years ago, according to a UN estimate.
Oil region Al-Shaddada is a town to the south of Hasaka, capital of the province of the same name, which accounts for most of the country's oil output.
On the video, heavily armed fighters dance and chant Islamist slogans, brandishing a torn poster of President Assad, which is beaten before being trampled underfoot.
At least one bloodstained body, of a man in military uniform, can be seen on the ground.
According to the Observatory, five of the Nusra fighters who died were from Kuwait.
"Dozens of employees" working for the state oil company were also killed in the assault on the town, it added without giving details.
Another organisation, the Syrian Human Rights Network based in Qatar, confirmed for BBC News that al-Nusra had captured al-Shaddada.
Reuters news agency quoted a spokesman for the main Syrian rebel group, the Free Syrian Army, as saying al-Nusra had overrun state security and military intelligence compounds in al-Shaddada.
The spokesman, Abu Laila, said an army garrison guarding a nearby gas field, al-Ghabsa, had fled as armed opposition units advanced towards al-Shaddada.
There were no immediate reports about the situation in al-Shaddada on Syrian state media.