Clintonjoined delegates from dozens of European, Arab and other countries to look at ways of supporting opposition representatives inIstanbuland to apply further pressure on President Bashar al-Assad's regime.
The chiefUSdiplomat told delegates that Assad, who has launched new assaults on Syrian towns and cities, was reneging on his commitment to a six-point peace plan he gave to UN-Arab League envoy Koffi Annan.
"Nearly a week has gone by, and we have to conclude that the regime is adding to its long list of broken promises,"Clintonsaid, according to prepared remarks distributed by the State Department.
Clinton also called for tighter regime sanctions, backed opposition efforts to forge a democratic and pluralistic vision for a future Syria, and announced an additional $12 million (nine million euros) in humanitarian aid, bring to $25 million the country's total contribution so far.
Clintonalso saidWashingtonis sending other aid, including communications equipment to help "activists organise, evade attacks by the regime and connect to the outside world."
But opposition Syrian National Council President Burhan Ghalioun, whomClintonwas to meet later, has asked the conference to back arming the rebels, a stand endorsed bySaudi ArabiaandQatar.
TheUSsaid it would work with international partners to gather evidence that can be used to hold perpetrators of abuses to account.
TheUSwill "train Syrian citizens working to document atrocities, identify perpetrators, and safeguard evidence for future investigations and prosecutions,"Clintonsaid.
"Stop killing your fellow citizens, or you will face serious consequences," she added.
UN officials estimate more than 9,000 people have died since Assad began crushing pro-democracy protests last March inspired by revolutions that have swept from power the leaders ofTunisia,Egypt,LibyaandYemen.
Clintonalso called for intensifying pressure from an array of US, European, Canadian, Arab and Turkish sanctions onSyria.
She said she was pleased the conference has "agreed to form a sanctions working group to coordinate and expand our national sanctions and strengthen reinforcement."
"Together we must must further isolate the regime, cut off its funds and squeeze its ability to wage war on its own people," she said.
To make the sanctions more effective, a US official speaking on condition of anonymity said, Clinton wants to establish a "clearing house of information on who is shipping arms, money to Assad to assist him in his killing" or evading sanctions.
She told delegates that she and her Gulf Arab counterparts who met inRiyadhon Saturday are urging Annan to produce a "timeline for next steps" if Assad fails to stop the bloodshed in line with the peace plan.
Annan's plan calls for a commitment to stop all armed violence, a daily two-hour humanitarian ceasefire, media access to all areas affected by the fighting, an inclusive Syrian-led political process, a right to demonstrate, and release of arbitrarily detained people.
Annan was due to brief the UN Security Council on Monday on the status of his plan.