Putin walks past Obama at the G20 in St. Petersburg
Russian President Vladimir Putin (L) walks past U.S. President Barack Obama (C) during a group photo at the G20 Summit in St. Petersburg September 6, 2013. At top left is British Prime Minister David Cameron. Reuters/Kevin Lamarque

US senators have advised sending defensive weapons to Ukraine so the latter can fight off pro-Russian rebels as President Vladimir Putin dangled the threat of nuclear war.

Robert Menendez, head of the Senate Foreign Relations Committee, Senator John McCain and Representative Mike Rogers, chairman of the House Intelligence Committee, all believe the U.S. must help Ukraine in its sovereignty war against Russia.

Menendez told CNN Ukrainians must be provided with defensive weapons that will make a dent on Russia's ambition to seize the country.

More importantly, give Ukraine the weapons they need so they can fight, McCain told CBS's Face the Nation.

On Friday, Ukraine's volunteer fighters expressed dismay over their seeming non-winning stance against the enemy. They said they can hardly sustain the fight against the better equipped pro-Russian rebels what with the latter's armaments and military tanks compared to their "small arms and a few RPGs (rocket propelled grenade launchers)."

But Mr Putin warned Ukraine's defenders to lay easy on meddling with their affairs, lest Russia be triggered to unleash its most important weapon.

"I want to remind you that Russia is one of the most powerful nuclear nations," the Russian president said. "This is a reality, not just words."

Speaking at a youth forum, Mr Putin said Moscow does not have intentions to go into "large-scale conflicts." But a report by state-run Itar-Tass quoted him telling the same audience that Russia is "strengthening our nuclear deterrence forces and our armed forces" to make them more efficient and modernised.

Thus, "it is better not to come against Russia."

Ukrainian volunteer battalions said the enemy fighters have dug up artillery positions in eastern Ukraine ready to fully seize the region. NATO, based on satellite images, alleged there are over 1,000 Russian soldiers already inside Ukraine. Britain, meantime, said Russia has moved 4,000 to 5,000 military personnel into the sovereign nation.

Latest data released by the United Nations revealed at least 2,593 people have died in war-torn eastern Ukraine since the conflict began in April. An average 36 people die each day.

Meantime, leaders of the European Union threatened Russia with a new round of maximum sanctions if it does not back down on its intentions to invade and seize Ukraine.

The sanctions, however, won't be really a new list but more of tightening the restrictions already imposed in July on Russia's financial, energy, and defence sectors.

"It's totally unacceptable that there are Russian soldiers on Ukrainian soil," British Prime Minister David Cameron said. "If [Russia] carries on in this way, the relationship between Europe and Russia, Britain and Russia, America and Russia will be radically different in the future."