U.S. President Joe Biden said on Friday he is putting together a set of comprehensive initiatives aimed at making it difficult for Russian President Vladimir Putin to move against Ukraine.

Biden, answering questions from reporters after a White House speech about the U.S. economy, said he had not yet spoken to Putin. A call between the two leaders is anticipated in days.

Ukraine’s defense minister, Oleksii Reznikov, warned earlier on Friday that Russia has massed more than 94,000 troops near Ukraine’s borders and may be gearing up for a large-scale military offensive at the end of January.

“What I am doing is putting together what I believe to be the most comprehensive and meaningful set of initiatives to make it very, very difficult for Mr. Putin to go ahead and do what people are worried he’s going to do,” Biden said.

He told reporters that his administration had been in “constant contact” with Ukraine and European allies about the situation.

A senior administration official, asked what Biden was talking about, said: “Since the beginning of this Administration we have demonstrated that the United States and our allies are willing to use a number of tools to address harmful Russian actions, and we will not hesitate from making use of those and other tools in the future.”

For its part, Moscow has accused Ukraine and the United States of destabilizing behavior and suggested that Kyiv might be preparing to launch its own offensive in eastern Ukraine, which the Ukrainian authorities strongly deny.