WASHINGTON, March 5 (CNN) - Russia is poised to deploy up to 1,000 more mercenaries to Ukraine in the coming days and weeks, as a senior Western intelligence official warned Moscow could "bombard cities into submission," an escalation that could lead to significant civilian casualties.

The US has already seen "some indications" that Russian mercenaries may be involved in Moscow's invasion of Ukraine "in some places," a senior defense official said earlier this week, but it wasn't clear exactly where or in what numbers.

"We've seen some indications that they're being employed," the official said.

Now, a US official tells CNN that Russia is planning to deploy up to 1,000 more mercenaries in the near future.