US President Barack Obama says that the United States is concerned about Russia's next move regarding the crisis in Ukraine.
"We are also concerned about further encroachment by Russia into Ukraine. So what I announced, and what the European Council announced, was that we were consulting and putting in place the framework, the architecture, for additional sanctions, additional costs, should Russia take this next step."
He said that in view of the concern that Russia might move further on Ukraine, the US was working alongside the European Council to create a framework for additional sanctions and costs should that happen.
Obama made the comment before his arrival in Brussels ahead of a EU-US summit.
At the same time, Ukrainian Prime Minister Arseny Yatseniuk says that his country would "fight" if Russian forces were to move into southeastern Ukraine.
"What's going to happen in case Russia crosses the border of the mainland -- this is the duty of every Ukrainian citizen, to protect our country. We will fight."
Lawmakers in Ukraine accepted the resignation of the country's defence minister as thousands of troops began withdrawing from the Crimean Peninsula.
Parliament has already approved a replacement.
Russia formally annexed Crimea on March 21, five days after a referendum that yielded an overwhelming vote to join Russia.
The Russian Federal Migration Service says that more than 6,500 Russian passports have been issued in Crimea whilst over 20,000 Crimean residents have applied for Russian passports.
The Russian Federal Migration Service is hoping to provide Crimeans with Russian passports in three months' time.
