Tuesday's emergency meeting is being held to address the crisis in Aleppo, following reports that Syrian forces executed dozens of civilians in the city.
"I formally and 100 percent reject allegations of revenge and summary executions," said Bashar Jaafari, Syria's ambassador to the UN.
"The actions of the Syrian government are the expression of the constitutional and legal right of the government to protect its citizens from terrorism."
Jaafari criticised UN members for quoting the "unverified testimonies" of civilians in Aleppo, before proceeding to show numerous and unverified photos of soldiers helping civilians as a means of "counter-testimony".
"This is not the fall of Aleppo, this is the liberation of Aleppo," he said.
France and Britain initially requested the meeting scheduled for midday (1700GMT) as the battle for Syria's second city neared an end, a turning point for the six-year war.
French Ambassador Francois Delattre called for action to confront "the worst humanitarian tragedy of the 21st century unfolding before our eyes."
"Aleppo is experiencing its darkest days," he told reporters.
"We have credible reports of brutal murders of families, summary executions, including women and children, houses put on fire with people trapped inside, continuing targeting of hospitals and medical staff, and the list goes on and on," he said.
UN Secretary-General Ban Ki-moon is to brief council members after UN rights officials said forces loyal to President Bashar al-Assad had entered homes and killed civilians inside.
Some civilians managed to flee the advancing forces but others "were reportedly caught and killed on the spot and others were arrested," said a UN spokesman in Geneva.
Delattre said Russia, Assad's ally, "holds the key of an exit strategy, a way out" of the crisis in Aleppo and stressed that Moscow "can't let this happen".
French Foreign Minister Jean-Marc Ayrault said in Paris that if the allegations of targeting of civilians were true, they would amount to war crimes or crimes against humanity.
"An investigation must be opened as soon as possible, carried out by the United Nations," said Ayrault.
Syria's army has taken more than 90 percent of the territory once held by rebels in east Aleppo, after launching an all-out offensive last month to seize control of the entire city.
"What a tragic day for Aleppo," said British Ambassador Matthew Rycroft.
"Even wars have laws, even wars have rules and every single rule, every single law of war has been broken by the regime," he said.