(CNN) -- Nearly 700 people have been killed in nine days of fierce clashes between an al Qaeda affiliate and other Islamist and rebel groups, activists said Sunday.
Forces from the al Qaeda-affiliated Islamic State of Iraq and Syria have increasingly come into conflict with Free Syrian Army fighters and other hard-line factions opposing the Syrian regime, while ISIS attempts to impose its strict form of Islamic Sharia law on areas coming under its control in northern Syria.
In the last nine days, 697 people have been killed in the fighting, activists said.
The victims include 351 combatants from the Islamist and non-Islamist rebel battalions, 246 ISIS fighters, and 100 civilians, the Britain-based Syrian Observatory for Human Rights said.
The activist group said the deaths took place between January 3 and January 12.
Among the 100 civilians killed in the clashes, 21 were executed by ISIS in the children's hospital in Qadi Askar in the northern rebel stronghold of Aleppo, the group said.
CNN cannot independently verify daily death tolls, but the United Nations has said more than 100,000 people have been killed in Syria since 2011.
Foreign ministers meet
The violence came as the "Friends of Syria" group of foreign ministers, including U.S. Secretary of State John Kerry, convened in Paris on Sunday in a last-ditch attempt to persuade the Syrian opposition to attend a peace conference in Geneva, Switzerland, at the end of January.
The Western-backed rebels are deeply divided on whether to go and will announce a decision on January 17.
"There is no military solution to the violence," Kerry said Sunday about a proposed new round of peace talks. "There is no other alternative to saving the nation of Syria than negotiations."
Kerry said he has "no expectations" about the success of the so-called Geneva II talks, but that the sides need to enter the meetings with the goal of "waging an even stronger effort to provide for this political solution."
"I am confident the Syrian opposition will come to Geneva," Kerry said.
Kerry added that Russian leaders have assured him they'll attend.
In a statement, the alliance of mainly Western and Gulf Arab countries called on armed groups to "respect democratic and pluralistic values" and allow humanitarian access.
It condemned the presence of foreign fighters in Syria, "both those fighting with the regime such as Hezbollah and other Iranian backed forces, and those fighting within other extremist groups," and demanded their immediate withdrawal.
It urged democratic opposition forces to keep opposing groups affiliated with al Qaeda.
"We fully support the Supreme Military Council of the Free Syrian Army and other democratic opposition forces in their action against the Islamic State of Iraq and the Levant," it said. The Islamic State of Iraq and the Levant is another name for ISIS.
For months, the rebel groups maintained an uneasy alliance as they fought to topple Syrian President Bashar al-Assad's government.
But now, the infighting has threatened to tip the balance among rebel forces toward militant groups and away from more secular brigades.
In a statement, the opposition Syrian Coalition media office condemned "any and all violations" and said armed gangs were taking advantage of the infighting between ISIS and the Free Syrian Army.
The statement, signed by Syrian Coalition media office director Khalid Saleh, cited these as "illegal practices, intimidating civilians, and theft, taking place all over Syria; particularly in the northern parts of the country."
"We call on FSA brigades to work with civil revolutionary bodies and local councils operating in those areas to address the threat of those gangs, and make sure those gangsters are hunted down and brought to justice," the statement said.
In Iraq, where at least nine people were killed and more than 40 wounded in several car explosions on Sunday, security forces in Mosul said they have detained 137 ISIS suspects in a series of raids in the city over the past seven days.
U.N. official in Damascus
On a visit to Damascus on Sunday, the United Nations humanitarian chief expressed concern for communities cut off by the months of fighting between government and rebel forces.
"I am particularly worried about the reports of starvation," Valerie Amos said in a statement after meeting with government officials as well as humanitarian organizations.
"The world must do more for all the people who are displaced. Many families are living in abandoned buildings, schools or in makeshift shelters, without enough food, clean water or medicine. We must help them to get through this very cold winter," she said after visiting a shelter in rural Damascus.
Amos recognized steps taken by the government to approve visas so aid can get in.
"But we need to do more in a crisis of this magnitude," she said.
CNN's Marie-Louise Gumuchian, Matt Smith and Samira Said contributed to this report.