Annan left Syria on Sunday without a deal to end the bloody year-old conflict there as President Bashar Assad's forces mounted a new assault on rebel strongholds in the north.
Annan met with Turkey's Prime Minister Recep Tayyip Erdoğan in Ankara on Monday, following talks in Qatar earlier in the day. Annan is expected to meet members of the Syrian opposition in Ankara on Tuesday, according to his staff.
Syrian activists said Monday that pro-government gunmen have killed at least 16 people - including children - in a rebel stronghold recaptured by the government in the embattled central city of Homs.
The UN estimates that Syria's crackdown has killed more than 7,500 people so far. The killings add to the pressure on UN Security Council members who are meeting to decide what to do next to stop the violence. The international community's current effort - a peacemaking mission by Annan - is faltering, with both the Syrian government and the opposition refusing to talk to one another.
"There are grave and appalling reports of atrocities and abuses. The killing of civilians must stop now. The world has to send a clear and united message in this regard," Annan told reporters upon his arrival in Ankara.
He said the diplomatic process would take time.
"This a very complex situation," said Annan. "We are going to press ahead for humanitarian access, for the killings of civilians to stop, and that get everybody to the table to work out a political solution."
UN Secretary-General Ban Ki-moon, meanwhile, urged Syria's president Monday to take swift action to end his regime's bloody crackdown and appealed to the divided UN Security Council to speak with one voice and help Syria "pull back from the brink of a deeper catastrophe." Ban said the conflict in Syria has led the entire region into uncertainty and subjected citizens in several cities to disproportionate violence.
Russia, which is Syria's most powerful ally, and China have vetoed two U.S. and European-backed Security Council resolutions that would have condemned Assad's bloody crackdown, saying they were unbalanced and demanded that only the government stop attacks, not the opposition. Moscow accused Western powers of fueling the conflict by backing the rebels.
The UN chief said he joined Annan "in urging President Assad to act swiftly, within the next few days in response to the proposals."
Syria's envoy to the United Nations in Geneva accused Israel of supplying arms to the Syrian rebels. Ambassador Fayssal al-Hamwi told the UN Human Rights Council on Monday that Israeli weapons were being funneled to opposition groups and foreign fighters linked to al-Qaida.
Israel said the allegation by the Syrian envoy was baseless.
"There isn't even one milligram of credibility to anything any Syrian official says these days," Israeli foreign ministry spokesman Yigal Palmor told The Associated Press. "This latest allegation carries even less credibility than the usual lies."
Al-Hamwi was responding to a UN-commissioned expert report that found "widespread and systematic violations" against civilians in the Syrian government's violent crackdown on opposition groups.
The UN panel's chairman, Brazilian professor Paulo Sergio Pinheiro, said civilians have died due to the government's obstruction of humanitarian aid.
The Red Cross is still waiting for unimpeded access to areas affected by fighting.