At least 800 people have been killed in a key Syrian city over the past 40 days, a Syrian monitoring group said Sunday.

Most of those killed were ISIS militants and Syrian Kurdish fighters, battling for control of Kobani, according to the Syrian Observatory for Human Rights.

The monitoring group based in London broke down the casualty figures starting September 16, the day ISIS began its attack on the Syrian Kurdish enclave.

More than half of the 815 people killed -- 481 -- were fighters with ISIS, which calls itself the Islamic State, the Syrian Observatory said in a statement.

They were killed in ambushes and suicide car bombings as well as in attacks led by a Kurdish female commander, the group said.

The group said 302 Syrian Kurdish fighters were also killed.

Two civilians beheaded

Civilians accounted for 21 of the dead; two of them were beheaded, according to the statement.

The real death toll could be a lot higher.

The group said it believes the actual casualty figure is twice the number documented, "because there is absolute secrecy on casualties and due to the difficulty of access to many areas and villages that have witnessed violent clashes and bombardment by the two sides."

Video from Kobani on Sunday showed a plume of smoke rising above the city, known in Arabic as Ayn al-Arab.

Most of the city's 200,000 people have fled across the border into neighboring Turkey.

New York Times: ISIS hostages were tortured before beheadings

U.S. military strikes

Word of the casualties comes as U.S. military forces continue to strike ISIS targets in Syria.

The U.S. military has conducted five airstrikes near Kobani since Saturday, according to a statement released by U.S. Central Command on Sunday.

Separately, the United States and its allies have conducted some 12 airstrikes in Iraq since Saturday, according to the CENTCOM statement.

(CNN)