UN chief Ban Ki-moon warned that the search for peace in Syria was at a "pivotal moment" and expressed strong concerns of an all-out civil war, as a five-week-old truce was broken yet again.
A bomb rocked the Damascus neighbourhood of Qaboon during the night killing five people, the Syrian Observatory for Human Rights said on Tuesday.
The Britain-based group gave no further information on the deadly explosion but said powerful blasts were also heard overnight in several regions across the country, including central Hama and the coastal cities of Banias and Latakia.
On Monday, at least 54 people, including 31 soldiers were killed in the ongoing unrest that has shaken Syria since March last year, the watchdog said.
The Syrian crisis also spread to Beirut, the capital of neighbouring Lebanon, for the first time since the upheaval erupted in March 2011.
As NATO ruled out military action against the regime of President Bashar al-Assad, a spokesman for UN leader Ban said at a Chicago summit of the alliance that he was increasingly worried about the situation in Syria.
"The secretary general said we were at a pivotal moment in the search for a peaceful settlement to the crisis and that he remained extremely troubled about the risk of an all-out civil war," Ban's spokesman said.
On the ground, fierce fighting in an area between Aleppo and neighbouring Idlib province, in northwestern Syria, killed at least 18 soldiers and two army deserters, the Syrian Observatory for Human Rights said.
The Britain-based watchdog said nine army deserters were killed overnight as they retreated under cover of darkness from Jisr al-Ab village near the Damascus suburb of Douma.
Also in the Damascus area, troops fired on people at a funeral, the Observatory said.
Elsewhere five civilians were killed, including two in a bombing and military raid in central Hama province, one by unidentified gunmen in the nearby region of Homs, and two more in fighting between the army and rebels in coastal Banias.
London-based Amnesty International said in its annual report Tuesday the Syrian government may have committed crimes against humanity by using "lethal and other excessive force against peaceful protesters" and "torturing" detainees in its crackdown.
The latest violence comes after a rocket-propelled grenade exploded on Sunday near UN observers in Douma, and at least 48 people were killed elsewhere in the country.
No one was hurt in the Douma blast, which came as UN mission head Major General Robert Mood and peacekeeping chief Herve Ladsous were leading observers around the trouble spot.
NATO, which undertook a major air war in Libya to back rebels who fought Moamer Kadhafi's forces last year, said it has "no intention" of taking military action against Assad's regime.
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"We strongly condemn the behaviour of the Syrian security forces and their crackdowns on the Syrian population," NATO chief Anders Fogh Rasmussen said at a Chicago summit on Sunday.
"But again NATO has no intention to intervene in Syria."
NATO states have come under criticism for backing the air war in Libya but ruling out military intervention in Syria, where opposition demonstrators and badly outgunned rebels have been hammered by heavily-armed regime forces.
Sunday's blast followed several other close calls for the UN monitors since they deployed in Syria, where 266 observers are now on the ground according to Mood.
The regime and its opponents trade accusations after such attacks.
An Islamist group, the Al-Nusra Front, on Monday claimed responsibility for a weekend suicide car bomb attack in Syria's main eastern city of Deir Ezzor that killed at least nine people and wounded 100 others.
The Al-Nusra Front said "a suicide bomber rammed a car bomb against buildings of military security, and aviation information, causing deaths and injuries among members of the regime."
It said it was "determined to continue its operations to clean the land of the Alawites and end the injustice that strikes the Sunnis" in Syria.
Deadly violence linked to the conflict in Syria erupted in Beirut, with overnight street battles between pro- and anti-Syrian groups.
"During the night, groups of young men cut off the road in the Tareek el-Jdideh district and street battles followed," a security official said.
"Two people were killed and 18 were wounded," he said, adding machineguns had been fired and that the fighting had raged until about 3:00 am (2400 GMT).
The clashes broke out after reports emerged troops had shot dead an anti-Syria Sunni cleric, Sheikh Ahmad Abdul Wahid, when his convoy failed to stop at a checkpoint in north Lebanon on Sunday.
The cleric's killing followed a week of intermittent clashes that left 10 people dead in Lebanon's northern port city of Tripoli between Sunnis hostile to the Syrian regime and Alawites who support Assad.
A Lebanese judicial official said 21 soldiers, including three officers, were being questioned by military police in relation to the cleric's death.
A high-ranking member of Hezbollah, Lebanon's powerful Shiite militia and close ally of Damascus, hit out at the opposition on Monday accusing it of transforming the north of the country into a rear base for Syrian rebels.
"The opposition has intentionally dragged Lebanon into the Syrian crisis and transformed into into a corridor and base for armed Syrians," Sheikh Nabil Kaouk, told a rally in the eastern city of Baalbek.