UPDATE 1-Poland says it expects NATO's eastern flank to be reinforced

(Adds details, background)

WARSAW, Feb 24 (Reuters) - Poland expects a decision on reinforcing NATO's eastern flank after Russian forces invaded Ukraine, a Polish government spokesman said on Thursday.

"Now is the time to reinforce NATO's eastern flank. We expect such decisions to be taken," Piotr Muller said.

Poland has repeatedly called for a stronger presence of armed forces from the North Atlantic Treaty Organization (NATO) in eastern Europe because of Russia's increasingly assertive policies.

Muller said on public TV that Russian President Vladimir Putin wanted to rebuild a great Russian state whose borders would possibly reach Poland.

"That is why the danger ... is real and why these actions aimed at reinforcing the NATO army are indispensable."

Poland is now hosting more than 4,000 enduring rotational U.S. military personnel, the fourth-largest concentration of U.S. forces in Europe, the U.S. Secretary of Defence Lloyd Austin said during a recent visit to Warsaw.

Muller also said Polish diplomatic missions in Ukraine would remain open "as long as possible", and repeated the prime minister's calls for tough sanctions on Russia to be adopted immediately.

(Reporting by Pawel Florkiewicz and Anna Wlodarczak-Semczuk; Editing by Tom Hogue and Christian Schmollinger)