What Assad "is doing is outrageous, unconscionable, unacceptable, disgraceful, craven, it's horrendous. And we all know that. Everybody knows that," Kerry said.
The top US diplomat insisted Washington was "committed to try to make a difference in ways that we have chosen within the law that we believe are appropriate and permissible."
And he admitted to MSNBC television that it was challenging trying to work out how to put more pressure on Assad as the war is about to enter its fourth year with no end in sight.
President Barack Obama and his administration were constantly reviewing the options available, he said, even though Obama walked back from threatened military strikes over Assad's use of chemical weapons last year.
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"There are limits on the ability of any nation to just spontaneously go out and use force whenever it wants," Kerry insisted.
"There are laws you have to follow and there's a process. The fact is that unless the nation that you're considering invites you in, unless you're doing it as a matter of self-defense or unless you have a UN resolution, there are greater limits than what you're able to do," Kerry added.