The senior State Department official noted "the Syrian armed opposition is getting a variety of weapons from a variety of countries" for many months.
"They are hopeful that we are going to take steps to deter the regime from using chemical weapons. They have complained for many months that we have not taken military action," the official said.
He added the United States has had "fairly intense" discussions with opposition leaders about what military action was under consideration.
"We have been very explicit to the Syrian opposition that any military action we might make in response to the chemic weapons attack is going to be limited and very focused on establishing the deterrence. Do they all welcome that? No. Some would like us to do more than that, and they will be disappointed, therefore."
But inaction, the official said, would leave the opposition "extremely disappointed" and "more fearful than ever that the regime will use chemical weapons."
He predicted larger refugee flows out of Syria as a result which could further destabilize neighboring Lebanon, Iraq and Jordan.
"They will look at the United States and other members of the international community as less reliable and there probably will therefore be that the moderates within the oppositions will feel somewhat weaker and will boost the prospects of groups on the more extreme edges," the official said, adding that such a scenario on the ground cold hurt chances for a political solution.
Even as he led calls within the administration for tougher action against Syria, Kerry has sought to work with Russian Foreign Minister Sergei Lavrov on bringing the warring parties in Syria together for negotiations on a political transition.
The senior official Kerry would be discussing those efforts during his talks this weekend, although no date has been set yet and the parties have "some distance to go" before the talks could be held. The United States hopes military action, however limited, could change al-Assad's calculus and encourage him to negotiate.
Although Syria is sure to top the agenda during Kerry's meetings, aides say he will still stress the importance of nurturing the fledgling peace process.
After five months of shuttle diplomacy between Israelis and Palestinians, Kerry in July announced a resumption of direct talks between the two sides and tapped former U.S. Ambassador to Israel Martin Indyk as his Mideast peace envoy.
The parties have committed to try and reach a peace deal within nine months, although privately both sides have voiced skepticism that was possible.
While in London, Kerry will also meet with Palestinian President Mahmoud Abbas. Another senior State Department official traveling with Kerry said although the crises in both Syria and Egypt have distracted both the Israelis and Palestinians, they have also yielded important benefits.
Kerry is also expected to lobby EU foreign ministers to reconsider sanctions imposed this summer against Israel, which banned funding of some projects in the occupied West Bank over European opposition to continued settlement building there.