A Syrian activist group says two weeks of fighting between an al-Qaida-linked group and other rebel forces in Syria has killed more than 1,000 people.

The Britain-based Syrian Observatory for Human Rights, which has a network of activists around Syria, said Thursday that the fighting in northern and eastern Syria killed 1,069.

The clashes pitting the al-Qaida-linked Islamic State of Iraq and the Levant and other groups are the most serious among rebel forces since the Syrian conflict began in March 2011.

The Observatory said Thursday that the dead included 130 civilians — including 21 who were "executed" by "Islamic State" members.

The United Nations human rights office said extremist Islamic groups are committing a "soaring" number of executions in the country's north that could amount to war crimes.