Islamic State in Iraq and the Levant (ISIL) militants kidnapped at least 400 civilians when they attacked government-held areas in the eastern Syrian city of Deir al-Zor on Saturday, a monitoring group said.
The Syrian Observatory for Human Rights said on Sunday families of pro-government fighters were among those abducted.
"There is genuine fear for their lives, there is a fear that the group might execute them as it has done before in other areas," said the Observatory's head Rami Abdulrahamn.
Deir al-Zor is the main town in a province of the same name. The province links ISIL's de facto capital in the Syrian city of Raqqa with territory controlled by the militant group in neighboring Iraq.
Syria's state news agency SANA claimed earlier that at least 300 people, including women and children, had been killed during the attacks in Deir al-Zor, but it made no mention of people getting kidnapped.
Syria's government condemned the killings which it described as a "horrific massacre against the residents of Begayliya in Deir al-Zor".
A source close to the Syrian government side said on Saturday that some of those killed had been beheaded.
Reuters was unable to independently verify the reports.
ISIL has previously carried out mass killings following military assaults in Iraq and Syria, including the slaughter of 200 soldiers captured from the Tabqa airbase in Raqqa province, and hundreds of members of the al-Sheitat tribe in Deir al-Zor in 2014.
The group, in control of most of Deir al-Zor province, has laid siege since March on remaining government-held areas in the city of Deir al-Zor.
Residents are facing severe food shortages and sharply deteriorating conditions. Of those under siege in the city, 70 percent are women and children, and many have been displaced from their homes elsewhere and are living in temporary shelters.
$40 bln needed annually
Meanwhile, a UN-appointed panel said on Sunday that an estimated $40 billion is needed annually to help the rapidly growing number of people needing humanitarian aid as a result of conflicts and natural disasters -- and one possibility to help fill the $15 billion funding gap is a small voluntary tax on tickets for football games and other sports, concerts and entertainment events, airline travel, and gasoline.
The panel's report on humanitarian financing, launched later on Sunday by UN Secretary-General Ban Ki-moon in Dubai, says the world is spending around $25 billion today to provide life-saving assistance to 125 million people devastated by wars and natural disasters -- more than 12 times the $2 billion that was spent in 2000.
"We have an exponentially growing problem," said panel co-chair Kristalina Georgieva, the European Commission's vice president for budget and human resources. "The good news is that the world has never been so generous to people in need. The bad news is that never has our generosity been so insufficient."
The nine-member panel calculated that an additional $15 billion is needed annually to reduce suffering and ensure that no one in need dies or has to live "without dignity" for a lack of money.
"This is a lot of money, but not out of reach for a world producing $78 trillion of annual GDP," the panel said in the report to the secretary-general released Sunday.
The 31-page report focuses on three inter-linked solutions to address the widening financial gap: mobilizing additional funds, shrinking the need for aid, and improving the efficiency of humanitarian assistance.
The report says that today's massive instability and its capacity to cross borders, demonstrated by the flight of people from Syria and other conflict areas to Europe, "makes humanitarian aid a global public good that requires an appropriate fundraising model."
It recommends that at the first UN humanitarian summit, to be held in Istanbul in May, governments voluntarily sign up to "the successful model of a solidarity levy and create a steady flow of revenues for humanitarian action."
The report points to a small levy on airline tickets, initially proposed by France, which raised 1.6 billion euros between 2006 and 2011 from just 10 participating countries - Georgieva estimated this at $2.3 billion - to help fund diagnosis and treatment for HIV/AIDS, malaria and tuberculosis in low-income countries.
Georgieva said in a video briefing from Brussels before the report's release that the panel couldn't agree on the specifics of a levy because some members are against taxation.
But she said she is "more optimistic on a voluntary levy, especially combined with social responsibility."
The panel has talked about a small tax on "high-volume transaction businesses" like Uber, concerts, entertainment, movies and sports and has been talking to some "potential players" including FIFA, the governing body of world football, Georgieva said.
She said people probably wouldn't feel a five-cent or 10-cent addition to a ticket or a ride, but the money generated could have a major humanitarian impact.
The report also calls for governments with greater wealth to provide more aid, for the humanitarian community to "harness the power of business to deliver its key skills and capabilities" including by supporting the delivery of aid and creating jobs, and for Muslim countries, which are most affected by conflict, to use "Islamic social finance" to help meet humanitarian needs.
Georgieva said zakat, the annual donation many Muslims are religiously required to make as a basic tenet of the Islamic faith, raises between $300 billion and $500 billion a year, and earmarking just one percent of that for humanitarian aid would go "a long way" toward closing the $15 billion gap.
To shrink the need for aid, the panel calls for world leaders to commit to preventing and resolving conflicts and to increasing investments in reducing the risk of natural disasters. In addition to putting more money in these areas, it recommends that aid should focus on "fragile countries" and countries experiencing shocks due to conflicts or natural disasters.
The panel also recommends that humanitarian aid should follow people in need, not countries. Georgieva said this means, for example, that when people fleeing conflict move from Syria to Lebanon and Jordan, those middle-income countries can get access to grants and low-cost loans.
To improve aid delivery, the panel calls for an end to competition between aid organizations and between humanitarian and development agencies.
