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Syrian rebels accused government forces of bombing a hospital in the besieged city of Aleppo late Wednesday, killing at least 40 people as the conflict there ground on, CNN reported. 

Ralib al-Omar, a leader of the Yusif al-Asma rebel group, said the attack targeted the Dar al-Shifa Hospital and that the dead included two nurses. The Local Coordination Committees of Syria, a network of opposition activists, said a doctor was among the dead as well -- one of 108 people the group said were killed across the country on Wednesday.

Dar al-Shifa is one of the main sources of medical help for people in Syria's commercial hub. In video posted by opposition activists, the blast appeared to have hit the hospital's often-crowded front lobby.

Aleppo has been the scene of some of the heaviest fighting of the 20-month-old Syrian conflict since August, and an artillery shell had hit the hospital's maternity ward earlier this year.

An estimated 37,000-plus people have been killed since protests broke out against Syrian President Bashar al-Assad in March 2011. Al-Assad responded by turning the army and police on the demonstrations, resulting in a civil war that has so far killed more than 37,000 people, according to the opposition Center for Documentation of Violations in Syria.

Most of those are civilians, though about 3,000 of the dead are government troops, the center reported last week. The government-owned Syrian Arab News Agency reported a series of clashes between security forces and armed terrorist groups -- its euphemism for opposition fighters -- and intense fighting near the Syrian-Turkish border in recent weeks has pitted loyalist Syrian forces and fighters from the rebel Free Syrian Army.

CNN cannot confirm claims by the government or the opposition because of government restrictions that prevent journalists from reporting freely within Syria.

In a sign of a potential escalation of the conflict, Turkey asked its NATO allies for Patriot missiles Wednesday to bolster its air defenses against its southern neighbor. A letter to NATO included the "formal request" that the alliance send "air defense elements," according to a Turkish government statement that cited "the threats and risks posed by the continuing crisis in Syria to our national security."

The statement added that the NATO Council would convene "shortly" to consider the matter. NATO Secretary-General Anders Fogh Rasmussen said in a Twitter post that the request would be considered without delay.