WASHINGTON - President Obama on Friday declared the U.S. and its allies would use "every tool available" to stop the slaughter of innocent people in Syria, uttering his most forceful words to date in response to an increasingly grim crisis that has gripped the world. The president did not give specifics about what the U.S. or other countries would do to help. "It is absolutely imperative for the international community to rally and send a clear message to President (Bashar) Assad that it is time for a transition," Obama said after a meeting with Danish Prime Minister Helle Thorning-Schmidt. It is time for that regime to move on. And it is time to stop the killing of Syrian citizens by their own government. The president added that nations cannot afford to be "bystanders" as the killing continues. Obama spoke in Washington shortly after Secretary of State Hillary Rodham Clinton used strong language to denounce Russia and China for protecting Syria, and the president's language about the need for world unity was viewed as a similar condemnation of those two nations. Obama said he was encouraged by developments out of Tunisia on Friday, where more than 60 nations, in a unified bloc, asked the United Nations to begin planning a civilian peacekeeping mission that would deploy after the Assad regime halts its brutal crackdown on the opposition. The Tunisia meeting is the latest international effort to end the crisis, which began when protesters inspired by uprisings sweeping across the Arab world took the streets in some of Syria's impoverished provinces nearly a year ago to call for political change. Assad's security forces have responded with a fierce crackdown. The government blames the violence on Islamic extremists and armed gangs. The situation has grown increasingly militarized in recent months, with opposition forces increasingly taking up arms against the regime. The U.N. estimated in January that 5,400 people were killed in the conflict in 2011. Hundreds more have died since.