"It's fair to say that Assad has improved his position a little bit, yes. But he's still not winning. This is a stalemate," Kerry told CNN television in an interview on Wednesday.
But asked whether he believed that America's policy in the war-torn country, which has seen a mounting death toll in its three-year conflict, had failed, he replied "No."
"The policy in Syria is just very challenging and very difficult," he added.
Earlier this week, the State Department denied reports Kerry told US lawmakers in a private meeting that he believed it was time to change strategy in Syria, where over 100,000 people have died and millions have fled their homes.
"I don't want to make any excuse whatsoever. We want this to move faster. We want it to do better," Kerry told CNN.
"But the point I'm making is that diplomacy is tough, slogging, slow work and hard work."
January marked the deadliest month so far in the conflict with some 6,000 dead, and Kerry said the United States was "always in the process of reevaluating whether there's more we can do, should do."
Kerry pledged the administration would work with Congress as well as internally to try to figure out how to push the Russians to use their influence on Assad to help improve conditions on the ground.
Syria has been gripped by deadly unrest since 2011. According to reports, Western powers and their regional allies - especially Qatar, Saudi Arabia, and Turkey - are supporting the militants operating inside the country.
NTJ/NJF