"We have all the tools no-one can boast of... we will use them if necessary", he said, in what is seen as a reference to ballistic missiles and nuclear arms.
Ukraine's allies have stepped up the supply of weapons, with the US vowing to make sure Ukraine defeats Russia.
Western officials say Russia is being hampered in its efforts in the east.
Last week, Russia launched a major offensive to seize the Donbas region after withdrawing from areas around the capital Kyiv.
But according to one official, Russian forces are "finding it difficult to overcome the staunch Ukrainian resistance and they are suffering losses".
In another development, the European Commission has accused Russia of blackmail after Moscow cut off gas exports to Poland and Bulgaria.
The Commission's President, Ursula von der Leyen said it showed Russia's "unreliability" as a supplier.
The Kremlin said Russia had been forced into the action by the "unfriendly steps" of Western nations.
Gazprom's cut-off follows Poland and Bulgaria's refusal to pay for gas in Russian roubles - a demand made by President Vladimir Putin in March, which was designed to shore up the faltering currency battered by Western sanctions.