Two US officials said attacks could continue beyond tonight, with a senior administration official saying “this isn't over."

"What you’ve seen tonight is not the end of the US response," the senior administration official said. "They have built a lot of flexibility into the plan to allow for further strikes based on what they’ve hit tonight."

A US official said a big concern is how much more sophisticated Russia’s capabilities are now compared to last year. The source says they are "significantly enhanced" in terms of anti-strike and anti-aircraft capabilities.

Part of the calculation this week has also been gaming out how Russia will respond either in the region or around the world.

“We are watching what Russians do in the next 24 hours,” the official said.

There will be intelligence collection to see what Russians are up to in the wake of the strikes, such as turning on their systems or talking about retaliating.