A senior Syrian opposition activist and her journalist daughter have been found dead at their apartment in Istanbul, the Dogan news agency reported on Friday.

Friends raised the alarm after being unable to reach Aroubeh Barakat and her daughter Halla Barakat by telephone, the news agency said.

Turkish police then arrived at their apartment in the Uskudar district on the Asian side of Istanbul and found both women dead. Unconfirmed reports said that their throats had been cut.

Since the outbreak of the Syrian civil war in 2011, Turkey has become home to almost three million Syrian refugees, many of them opponents of the regime of President Bashar al-Assad.

Aroubeh Barakat's sister Shaza also confirmed the deaths in a Facebook post, saying the two "were assassinated at the hands of injustice and tyranny". She said that they had been stabbed to death.

She said her sister had opposed the Assad regime from the 1980s going back to the rule of Bashar al-Assad's father Hafez. 

The Yeni Safak daily said Aroubeh Barakat had carried out investigations into alleged torture in prisons run by the Assad regime.

It said she had initially lived in Britain, then the United Arab Emirates before coming to Istanbul.

Syrian activist Rami Jarrah wrote on Facebook that the family believes the killing could be due to Aroubeh's opposition activities.

Halla Barakat, 22, was working for a website called Orient News and had also for a time worked for Turkish state broadcaster TRT. 

Syrian opposition activists and journalists have repeatedly complained of threats to their security.

Two Syrian journalists from the city of Raqa who were opposed to the Islamic State (IS) extremist group and were found beheaded in southern Turkey in 2015.