A Syrian opposition activist and her daughter were found killed in their flat in Istanbul, relatives said.

Orouba Barakat, 60, and daughter Halla, 23, had been stabbed to death, according to a Turkish police official

Their bodies were wrapped in blankets and sprinkled with lime detergent to prevent odour, police said, as they investigated the scene at the flat in Uskudar, a district in the Asian part of the city, on Friday.

"It has been said their corpses were in the home for four days," a neighbour told Turkish daily Hurriyet.

"They were nice people and did not harm anyone."

Oroba Barakat was 1 of the reasons I studied journalism. She was the only woman journo I knew. Both stood up 4 the #Syria|n ppl. Both r gone pic.twitter.com/3kPmCNiqJn — Zaina Erhaim (@ZainaErhaim) September 22, 2017

The elder Barakat was a well-known and outspoken critic of Syrian President Bashar al-Assad. She was a member of the opposition group the Syrian National Coalition and had written extensively on the use of torture in government prisons.

She was also active in Turkey's Syrian community, and helped out refugees financially, as one recalled to the New York Times.

Orouba lived in exile since the 1980s in the UK, the US, Saudi Arabia and the UAE before moving to Turkey.

The coalition released a statement saying: "The hand of terrorism and tyranny is the prime suspect in this heinous crime of assassination."

Ahnad Ramadan, the head of the National Action Movement for Syria, tweeted that the coalition is following up with Turkey to conduct an investigation.

جريمة آثمة أودت بحياة الزميلة عروبة بركات وابنتها حلا باسطنبول. لهما الرحمة وللعائلة أصدق العزاء. نتابع مع الجانب التركي سير التحقيق. #سورية pic.twitter.com/ryzZNQSqih — أحمد رمضان A.Ramadan (@RamadanSyria) September 21, 2017

Translation: A sinful crime that claimed the lives of my colleague Orouba Barakat and her daughter Halla in Istanbul. May they have mercy. Deepest condolences to their family members.

Halla Barakat was an editor with the Dubai-based Orient News, owned by Syrian opposition figure Ghassan Aboud. Before that, she was a reporter with Turkey's TRT World Service.

The head of the Syrian Journalists Association Ali Eid called on the Turkish authorities to conduct a swift investigation in order to find and punish the culprit. He also called for added protection for journalists living in Turkey.

READ MORE: The case for - Can journalists be activists?

The Committee to Protect Journalists echoed Eid and in a statement said that "Turkey must ensure the protection of Syrian journalists who have fled to the country seeking safety."

"We call on Turkish authorities to find those responsible for the murders of Halla Barakat and Orouba Barakat, and bring them to justice," CPJ Europe and Central Asia Programme Coordinator Nina Ognianova said.

Family members said that the two Barakats had received threats in the days before they were murdered.

Orouba's sister Shaza wrote a Facebook post confirming the deaths.

"The hand of tyranny and injustice assassinated my sister Dr Orouba Barakat and her daughter Halla in their Istanbul flat," she wrote.

"We mourn our sister, the ardent fighter that the Baathist regime has pursued since the 1980s until she was finally assassinated in a foreign land."

READ MORE: Syria journalists 'on the margins of history'

Orouba was the aunt of Deah Barakat, who along with his wife and her younger sister, were killed in North Carolina in 2015 by their neighbour, in what came to be known as the Chapel Hill Shooting.

Deah's sister Suzanne expressed her shock and disbelief on social media.

"Khale (Aunt) and Halla were vocal activists in the Syrian revolution, speaking truth to power, and raising awareness about the atrocities committed by the Assad regime," she wrote on her Facebook.

"I'll always remember Halla, whose name means beauty, as the little girl with golden curls and bright green eyes. She grew to become a dynamic, educated, kind, fun-loving, social justice-oriented individual, just like her mom."

Syrian opposition activists and journalists have occasionally been targeted in Turkey in recent years.

In 2015, Naji Jerf, a prominent journalist and vocal critic of Assad and the Islamic State State of Iraq and the Levant (ISIL, also known as ISIS) group, was shot dead in Gaziantep.

In the same year, two Syrian journalists from the city of Raqqa who were opposed to ISIL were found beheaded in southern Turkey.