President Barack Obama has described the situation in Syria as "heartbreaking" but said that unilateral US military intervention there would be a mistake.
Obama said President Bashar al-Assad would fall, as other dictators had fallen, but the US would try to achieve this by working to isolate Syria.
In his first news conference of 2012, Obama warned his critics not to forget the cost of war.
He rejected a comparison to Libya, saying Syria was more complicated.
Meanwhile, US Senator John McCain - Obama's opponent in the 2008 presidential election - called for US air strikes against Syrian forces.
More deadly clashes were reported across Syria yesterday.
The international community remains divided on how to end violence there.
The UN says more than 7,500 people have died as a result of the violence in Syria over the past 12 months.