National Security spokeswoman Caitlin Hayden suggested Thursday that the United States was prepared to move ahead with or without U.K. support.
"As we've said, President Obama's decision-making will be guided by what is in the best interests of the United States," Hayden said.
"He believes that there are core interests at stake for the United States and that countries who violate international norms regarding chemical weapons need to be held accountable."
Such comments stand in contrast to the tone senior officials took last week, saying repeatedly that the United States would only act in coordination with its allies.
Defense Secretary Chuck Hagel said this morning the United States was still on the hunt for a coalition.
"Our approach is to continue to find an international coalition that will act together," Hagel told reporters in Manila, the Philippine capital. "And I think you're seeing a number of countries state, publicly state, their position on the use of chemical weapons."
Hagel added, "Every nation has a responsibility to make their own decisions, and we respect that of any nation."
The defense secretary is expected to return to Washington later today.
After Hagel's comments, France hinted that it might follow the United States' lead.
French President Francois Hollande told Le Monde in an interview published today that he supported "firm" punitive action against the Syrian regime.
"Each country is sovereign to participate or not in an operation," Hollande told the French newspaper. "That is valid for Britain as it is for France."