(CNN) -- The Tuesday deadline for Syria to withdraw its military forces from towns and cities is "not an excuse for continued killing," the spokesman for U.N. Secretary-General Ban Ki-moon said Friday.
Despite the agreed-upon deal, Syria continued attacks in various parts of the country, opposition groups reported.
"The 10 April timeline to fulfill the government's implementation of its commitments, as endorsed by the Security Council, is not an excuse for continued killing," the spokesman said.
Speaking for Ban, he said the secretary-general is concerned at the "rapidly deteriorating humanitarian situation" in Syria, which is now affecting more than 1 million people.
"The Syrian authorities remain fully accountable for grave violations o
f human rights and international humanitarian law. These must stop at once," he said.
At least 52 people were killed around the country Friday, the opposition Local Coordination Committees of Syria said.
The regime of President Bashar al-Assad shelled homes in the Damascus suburbs, seemingly at random, the group said.
The military action appears to bolster the view of those who doubt al-Assad's commitment to a peace plan championed by Kofi Annan, the former U.N. secretary-general who is the point person on Syria for the United Nations and Arab League.
A deal he brokered -- and that al-Assad said he agreed to -- calls for a cease-fire by the government and the opposition, and a Syrian-led political process to end the crisis. It also calls for Syrian troops to leave many population centers, but troops remained in many of those places Friday.
In the Damascus suburb of Daraya, random shelling damaged a home, the LCC said. In the suburb of Douma, there were reports of "continued firing of missiles from tanks" toward homes, the group said.
Instead of withdrawing, the military was also active in the suburb of Harasta, where heavy clashes between the army and the opposition Free Syrian Army were reported.
CNN cannot independently verify opposition and government claims, as al-Assad's regime has severely restricted the access of international journalists.
Syria has been engulfed in violence since March 2011, when government forces began a brutal crackdown on a protest movement calling for reforms that quickly devolved into an uprising demanding the ouster of the regime.
The government has consistently blamed "armed terrorist groups" for the violence, but most reports from inside the country suggest it is pummeling neighborhoods in an attempt to wipe out dissidents.
World powers have been working to stop the fighting, which the United Nations estimates has killed at least 9,000 people. The LCC puts the toll at more than 11,000 people.
Of those killed Friday, the LCC said at least 28 died in the opposition stronghold city of Homs, which has been the scene of some of the most intense clashes in recent months.
In nearby Rastan, government forces were reportedly firing rockets at opposition strongholds within the city, the opposition Syrian Observatory for Human Rights said.
Shelling was also reported in Andan in the Aleppo province, with Syrian forces raiding the homes of suspected opposition members, the LCC reported. The group said that at least 12 people died there.
Factional fighting among civilians was also reported Friday, further challenging the viability of a cease-fire agreement.
Fierce fighting erupted between armed men loyal to al-Assad and rebels from opposing villages.
At least two women were killed and four were injured in fighting that broke out between military defectors and armed men loyal to al-Assad in the beleaguered western province of Homs, the Syrian Observatory said.
Clashes in the town of Hula reportedly occurred between military defectors from one nearby village and armed men loyal to the regime from two other villages, the opposition group said.
The reports of fighting between villagers with opposing loyalties highlight the divisions inside Syria, a nation with a majority Sunni population that is governed by al-Assad -- a minority Alawite, which is an offshoot of Shiite Islam.
Some analysts have expressed concern about what the Sunni-dominated Muslim Brotherhood might do if al-Assad's Alawite-dominated regime falls.
The Syrian government has said it will implement the peace plan, saying it has taken steps to comply. Its ambassador to the United Nations, Bashar Jaafari, acknowledged that fighting was still going on -- but he blamed that on opposition groups he said were being armed by Saudi Arabia, Turkey and Qatar, which currently holds the presidency of the U.N. General Assembly.
The government has committed to the April 10 deadline but is demanding a guarantee from Annan that once its troops pull back, other groups will do the same.
CNN's Ivan Watson, Salma Abdelaziz, Kamal Ghattas and Yesim Comert contributed to this report.