Any country trying to intervene in the Ukraine war will face a "lightning-fast" response, Russian President Vladimir Putin has warned.

"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.

Mr Putin made his comments speaking to Russian lawmakers in the northern city of St Petersburg on Wednesday.

"If someone from the outside tries to intervene in Ukraine and create strategic threats for Russia, our response will be lightning fast," he said.

"We have all the tools [to respond] that no one can boast of. And we will not be bragging about them, we will use them if necessary."

The Russian leader added that all decisions on what that response would include had already been made - without providing any further details.

Russia invaded Ukraine on 24 February, and within days President Putin ordered his military to put Russia's nuclear deterrence forces on high alert.

Analysts suggest such threats are an attempt by Mr Putin to warn Ukraine's allies not to intervene more in the conflict.

President Putin was speaking a day after Western nations held a summit in Germany, promising to ramp up military support for Ukraine.

US Defence Secretary Lloyd Austin pledged to move "heaven and earth" to make sure Ukraine won the war.

There has recently been an increasing number of pledges to increase military support to Ukraine, including Germany's announcement that it will send 50 anti-aircraft tanks, in a sharp U-turn in policy.