After a month-long campaign in Syria, Arab and U.S.-led warplanes have killed 521 Islamic State fighters and 32 civilians since airstrikes against the terrorist organization began, a monitoring group which tracks the violence said on Thursday. The real numbers, however, could be much higher.

Bombing raids were responsible for the vast majority of ISIS militant deaths, 464, near the northern Syrian town of Kobane, the target of a massive jihadi offensive, the Britain-based Syrian Observatory for Human Rights stated, adding that civilian deaths included six children and five women, Reuters reported.

Another 57 fighters with the al-Qaida-linked Nusra Front were killed in airstrikes on the northern province of Aleppo and Idlib, the Observatory said.

Additionally, more than 200,000 people have been forced to flee for safety in neighboring Turkey after dozens of Kurdish villages were captured by ISIS fighters.

Citing Article 51 of the U.N. Charter, which covers an individual or collective right to self-defense against armed attack, Washington offered justification for actions that resulted in civilian deaths in the Syrian provinces of Aleppo, Deir al-Zor, Idlib, Raqqa and al-Hassakah over the weekend.

U.S. Central Command spokesman Colonel Patrick Ryder said on Saturday that Washington took "reports of civilian casualties or damage to civilian facilities seriously and we have a process to investigate each allegation."

Attacks on oil facilities in Syria, known to be a key source of income for militants, have not only endangered civilians, but they have also led to a sharp rise in the price of oil products in rebel-held areas of Syria, according to UK MailOnline.

ISIS is an al-Qaeda offshoot which has grabbed large areas of Syria and neighboring Iraq. Although air strikes against the group in Iraq had been ongoing since July, aggressive war activity in Syria only began in mid-September by an international coalition of U.S., Britain, France, and with the help of Arab allies.

Earlier this week, a report confirmed that more than 135 airstrikes had been fired against ISIS militants in and around Kobane, killing hundreds of fighters, the U.S. Central Command said.

"Combined with continued resistance to ISIL on the ground, indications are that these strikes have slowed ISIL advances into the city, killed hundreds of their fighters and destroyed or damaged scores of pieces of ISIL combat equipment and fighting positions," Central Command said in a statement.

Meanwhile, the three-year civil war in Syria has caused close to 200,000 deaths, according to the United Nations.