"We are all here because the Assad regime's escalating violence in Syria is an affront to the international community, a threat to regional security and a grave violation of universal human rights," Clinton said at the international Friends of Syria conference in Tunis.
"The Assad regime has ignored every warning, squandered every opportunity and broken every agreement," she said. "Faced with determined protesters demanding their rights and their dignity, the regime is creating an appalling humanitarian disaster."
Clinton said the United States strongly supports the Arab League's demand that Syrian forces "immediately halt all attacks against civilians" and called for a "negotiated political solution to the crisis."
The Tunis meeting came as the United Nations named top international diplomat Kofi Annan, a former U.N. secretary-general, as an envoy to Syria and targeted top Syrian officials for "gross human-rights violations."
Annan -- secretary-general from 1997 to 2006, and a Nobel Peace laureate with the United Nations in 2001 -- was named a special U.N.-Arab League envoy, with a mandate to try to end the bloodshed and arrange a political transition, the world body said.
Annan, from the West African country of Ghana, is said to have kept good lines of communications with the Syrian regime, The Wall Street Journal reported.
U.S. President Barack Obama, meeting with Danish Prime Minister Helle Thorning-Schmidt in Washington, said: "All of us who have been seeing the terrible pictures coming out of Syria and Homs recently recognize it is absolutely imperative for the international community to rally and send a clear message to President Assad that it is time for a transition. It is time for that regime to move on and it is time to stop the killing of Syrian citizens by their own government. …
"It is important that we not be bystanders during these extraordinary events," he said.
The Red Cross said Friday it has begun evacuating injured women and children from Homs, Syria, after weeks of shelling by the regime of President Bashar Assad.
Homs has been bombarded by government forces for weeks, causing death, destruction and shortages of much-needed food and medicine, even as Assad denies his troops are targeting civilians, CNN reported.
Syrian Arab Red Crescent ambulances began removing the sick and injured from the hard-hit suburb of Baba Amr after negotiating with the government, the BBC said.
"The ICRC and the Syrian Arab Red Crescent are on the spot in Baba Amr, attempting to evacuate as soon as possible everyone in need of urgent help," International Committee of the Red Cross spokesman Saleh Dabbakeh told the British newspaper The Daily Telegraph in Damascus, adding at least 11 ambulances were active in the operation, which included four Western journalists.
The BBC reported the main opposition force in Syria asked the Friends of Syria conference to supply rebel fighters with weapons to use in combat against Syrian forces
Officials from the Syrian National Council, the leading opposition group, said other countries should be allowed to supply arms if Assad refuses to give up power.
"If the regime fails to accept the terms of the political initiative outlined by the Arab League and end violence against citizens, the Friends of Syria should not constrain individual countries from aiding the Syrian opposition by means of military advisers, training and provision of arms to defend themselves," the Syrian National Council said.
White House deputy press secretary Josh Ernest repeated the administration's position that "further militarizing the situation in Syria at this point in time is not a wise course -- that is not the wise policy course to pursue at this point."
The United States was working with its partners and other countries in the region "to see if there [are] things that we can do to bring humanitarian relief and aid to those who are affected by the violence, including medical supplies," Ernest said, "to see if there are things that we can do to offer support to the Syrian National Council to speed a democratic transition -- a political solution in Syria -- and to increase the pressure through sanctions on the Assad regime."
The deaths of two foreign journalists -- Marie Colvin, a U.S.-born correspondent for The Sunday Times of London, and Remi Ochlik, a French photographer -- have increased international anger at the Assad regime. French President Nicolas Sarkozy described their deaths as "murder" Thursday, The New York Times said.
The Syrian government issued a statement saying Colvin, Ochlik and journalists wounded in a rocket attack on a makeshift press center in Homs were in Syria without informing authorities and at their own risk.
The United States, Britain, France and Turkey are among the nations joining the first meeting of the Friends of Syria group.