The concerns were expressed in a joint statement by Francis Deng, special adviser to Secretary General Ban Ki-moon on the prevention of genocide, and Edward Luck, special adviser on the responsibility to protect.
"Based on available information, the special advisers consider that the scale and gravity of the violations indicate a serious possibility that crimes against humanity may have been committed and continue to be committed in Syria," they said.
They urged an "independent, thorough, and objective investigation" of the events in Syria and echoed calls by Ban to Assad's government to allow humanitarian access to areas affected by the unrest.
The London-based Syrian Observatory for Human Rights says 1,483 civilians have been killed since protests erupted March 15 in the autocratic Arab country. Rights groups say more than 12,000 people have been arrested and thousands have fled the country, many to Turkey and Lebanon.
On Saturday, troops backed by tanks entered villages in the Jebel al-Zawiya district of the northwestern province of Idlib near the Turkish border, the head of the Syrian Observatory for Human Rights told AFP.
"The tanks and armoured personnel carriers entered Sarjeh and neighbouring villages and the army is pounding caves dug into the mountains which (anti-regime) militants often use as hideouts," said Rami Abdel Rahman.
The army swept through Jebel al-Zawiya at the end of June to crush mounting anti-regime dissent there, arresting scores of people.
Security forces also arrested civilians in the flashpoint central city of Homs, where gunfire was also heard, Abdel Rahman and another activist, Abdel Karim Rihawi, told AFP in Nicosia.
Women were among the many civilians rounded up, they said.
"Gunshots were heard in Al-Khalidiyeh neighbourhood and security forces have been making arrests," said Rihawi, who heads the Syrian League for the Defence of Human Rights.
Abdel Rahman spoke of a "campaign of arrests in Homs that has targeted several women" and said six explosions were heard on Friday night near the Baba Amr neighbourhood of Homs, Syria's third city.
Security forces also made arrests on Saturday in the Damascus neighbourhood of Rukneddin, which has a mostly Kurdish population, Rihawi said.
On Friday, troops and security forces barricaded Rukneddin and the Qabun district of Damascus and conducted house-to-house searches, activists had said.
More than 50 people have been killed in central Homs in the past week, activists have said, accusing the regime of sowing sectarian strife among the city's Christians, Sunni Muslims and Assad's Alawite minority community.
Troops earlier this week arrested "armed men" in Homs and seized "stockpiles of weapons," the pro-government Al-Watan newspaper reported on Wednesday.
On Friday more than 1.2 million protesters swarmed cities in the north and east of the country in support of Homs, where 25,000 people also rallied, activists have said.
Security forces and pro-regime agents used force to disperse protests across Syria on Friday, killing eight people, including two in Homs and one in Idlib province, they said.
Activists said the authorities are trying to put the lid on anti-regime dissent before the start of Ramadan, when devout Muslims fast from dawn to dusk and meet in the evenings at mosques for special "taraweeh" prayers.
"They want to stifle dissent as soon as possible before Ramadan," one activist said, because authorities are worried that protests will gather pace when worshippers emerge from the "taraweeh" prayers.
Meanwhile a 12-year-old boy who was shot in the head earlier this month by a policeman as he took part in an anti-regime rally near Damascus died of his wounds on Saturday, activists said.
On Friday France and Britain denounced the deadly crackdown on protesters, particularly in Homs, with British Foreign Secretary William Hague saying he was "appalled" by the killings of civilians.