Syria - The United States appeared to open the door to eventually arming Syria's opposition, saying that if a political solution to the crisis was impossible it might have to consider other options.

The comments, made by officials at both the White House and the U.S. State Department on Tuesday, marked a shift in emphasis by Washington, which so far has stressed a policy of not arming the opposition and has said little about alternatives.

In Syria on Tuesday, activists said President Bashar al-Assad's forces killed more than 60 people in attacks on villages and an artillery barrage on the restive city of Homs. The Red Cross called for daily ceasefires to allow in urgently needed aid.

"We still believe that a political solution is what's needed in Syria," White House spokesman Jay Carney said.

"We don't want to take actions that would contribute to the further militarisation of Syria, because that could take the country down a dangerous path. But we don't rule out additional measures."

Asked if the United States was shifting its stance on arming the rebels, U.S. State Department spokeswoman Victoria Nuland said Washington did not want to see the violence increase and was concentrating on political efforts to halt the bloodshed.

"That said ... if we can't get Assad to yield to the pressure that we are all bringing to bear, we may have to consider additional measures," she said, declining to elaborate.

Secretary of State Hillary Clinton will meet representatives of some 70 countries in Tunis on Friday for the first "Friends of Syria" meeting to coordinate the international community's next steps in response to the nearly year-long uprising against Assad.

The United States and its allies hope the Tunis conference will allow them to begin drawing up a plan for Syria after Russia and China vetoed a Western-backed Arab League peace plan at the U.N. Security Council.