The West’s supply of weaponry to the Ukrainian army has significantly boosted its military efforts during the war and sparked fury in Moscow. Earlier this month, Russian officials told the White House that there would be “unpredictable consequences” if the US continued to send arms to Kyiv.
Mr Lavrov was asked about the importance of avoiding a global conflict and whether the current situation was comparable to the Cuban missile crisis in 1962, when the US and the Soviet Union stood on the brink of nuclear war.
“This is our key position on which we base everything. The risks now are considerable,” Mr Lavrov said.
“I would not want to elevate those risks artificially. Many would like that. The danger is serious, real. And we must not underestimate it.”
The Kremlin has consistently used the threat of nuclear war to dissuade the west from intervention in Ukraine, with Vladimir Putin in February warning that countries who meddled in the conflict faced “consequences you have never seen before”.
Western leaders have provided military support to Ukraine but categorically ruled out direct confrontation with Russia on the battlefield, citing a desire to prevent the conflict from escalating.
Watch: Russia's Lavrov warns of 'real' danger of nuclear war
Mr Lavrov said that western supplies if sophisticated weapons, including Javelin anti-tank missiles, were a “legitimate target” for Russia’s military.
“Nato, in essence, is engaged in a war with Russia through a proxy and is arming that proxy. War means war,” he added.
His comments came just days after Washington announced a fresh package of military aid for Ukraine, including an extra £234 million in foreign military financing and a £129 million sale of ammunition.
Speaking on a visit to Kyiv, secretary of state Antony Blinken said Moscow was failing in its war aims and a “sovereign, independent Ukraine” would be around “a lot longer than Russia’s Putin is on the scene”.
Ukrainian foreign minister Dmytro Kuleba said that Mr Lavrov’s comments underscored Ukraine’s need for Western help.
“Russia loses last hope to scare the world off supporting Ukraine. Thus the talk of a `real' danger of WWIII. This only means Moscow senses defeat in Ukraine,” he said.
After failing to take Kyiv and other central Ukrainian cities, the Kremlin has launched a fresh offensive in the Donbas region in the country’s east.
Last week, Rustam Minnekayev, a Russian military commander, said the Kremlin wants full control of southern Ukraine, which he said would open the way to Transnistria - a breakaway region of Moldova that sits along the Ukrainian border.