U.S. President Barack Obama has said Russia is acting "not out of strength, but out of weakness" in Ukraine. He also warned of the possibility of further sanctions against Russia if it encroached further into Ukraine, BBC News reports.

Speaking in The Hague, Obama said Tuesday, March 25, he was encouraged by the willingness of EU countries to consider sanctions that could possibly hurt them as well.

The U.S. has already imposed sanctions on Russian individuals after Moscow annexed Crimea earlier this month.

Obama, who is attending a meeting of G7 countries, after Russia was expelled, said the annexation was not a "done deal" as the international community had not recognized it but admitted that "the facts on the ground are that the Russian military controls Crimea".

He expressed concern about the large numbers of Russian troops massed on the border but did say Russia has the right to deploy troops on its own territory.

"It is up to Russia to act responsibly and show itself once again to be willing to abide by international norms and... if it fails to do so, there will be some costs," he said.

Obama said that certain sanctions discussed with European countries could "cause some disruptions to each of our economies, or certain industries" but said he was "encouraged by... the firmness and willingness on the part of all countries to look at ways in which they can participate in this process".

He added that he hoped the International Monetary Fund would quickly finalize an aid package for Ukraine, adding that it was important to help Ukraine hold successful elections in May.

Also on Tuesday evening, there were reports that Russian troops had seized the last remaining Ukrainian warship in Crimea.

A Ukrainian defense spokesman said a Russian-flagged tugboat was towing away the minesweeper Cherkassy.

The last military base under Ukrainian control in the region, in Feodosia, was seized by Russian forces on Monday.