WASHINGTON (AFP) - US Secretary of State John Kerry on Wednesday renewed allegations against Russia of helping the Syrian regime, accusing Moscow of increasing its aid to President Bashar al-Assad.

What Mr Assad "is doing is outrageous, unconscionable, unacceptable, disgraceful, craven, it's horrendous. And we all know that. Everybody knows that," Mr 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.

"Frankly, Russia is increasing its assistance to Assad. I do not find that constructive in the effort to try to get him to change his mind and be able to come to a decision that he needs to negotiate in good faith," Mr Kerry said.

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.

"There are limits on the ability of any nation to just spontaneously go out and use force whenever it wants," Mr 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," Mr Kerry added.

"I can tell you that none of us are satisfied, not the president, not me, no one in this administration is satisfied with where we are today. We believe we need to do more."