“The killing of civilians must end now. The world must give a clear message that this is unacceptable,” Annan, who is mediating on behalf of the United Nations and Arab League, said in comments carried by Turkey’s NTV channel.
In a brief statement at Ankara airport Annan said the situation in Syria was “complex.”
He urged Syrian authorities to grant free access to humanitarian aid for the civilian population in areas where anti-regime protests have flared up.
The bodies of 47 women and children, some with their throats slit, were found in the restive Syrian city of Homs after a “massacre” that sent families fleeing the area, activists and the opposition said on Monday.
And activists said that the Syrian army launched a new assault in the restive northern province of Idlib and the city itself, where residents are suffering “indescribable” humanitarian conditions.
In Geneva, the president of a special UN Human Rights Council inquiry into human rights in Syria said unimpeded humanitarian access should be the rule and not the exception.
Annan said he was confident that talks for a settlement of the year-old Syria crisis would eventually succeed.
“We will launch a political process and we will reach a settlement,” Anatolia news agency quoted him as saying, but he added that this “will not be easy.”
Annan left Syria empty-handed on Sunday after failing to secure an accord.
The UN-Arab League envoy departed from Damascus at the end of a two-day mission during which he said he presented President Bashar al-Assad with “concrete proposals” to halt unrest that monitors say has claimed more than 8,500 lives since March last year.
Annan was to inform Turkish Prime Minister Recep Tayyip Erdogan of his talks in Damascus and later confer with Foreign Minister Ahmet Davutoglu.
He was to leave Ankara on Tuesday morning but said he would not have time to visit Syrian refugee camps in southern Hatay province, where 12,605 Syrian have sought shelter, according to the latest Turkish figures published Monday.