Russian President Vladimir Putin has told Belarus that Moscow would supply Minsk with missile systems capable of carrying nuclear weapons, the Russian Foreign Ministry said.
During a meeting in St Petersburg, Belarus President Alexander Lukashenko said he was concerned about what he called the aggressive and repulsive policies of neighbouring Lithuania and Poland.
He asked Mr Putin to help Belarus mount a “symmetrical response” to what he said were nuclear-armed flights by the US-led Nato alliance near Belarus's borders.
Mr Putin said he saw no need at present for a such a response, but that Belarus' Russian-built Su-25 jets could, if necessary, be upgraded in Russian factories.
“We will transfer Iskander-M tactical missile systems to Belarus, which can use both ballistic and cruise missiles, both in conventional and nuclear versions,” a Foreign Ministry summary of the meeting quoted him as saying.
The Iskander-M, a mobile guided missile system, has two missiles with a range of up to 500 kilometres.
Parts of the meeting between the two leaders were televised.
“Minsk must be ready for anything, even the use of serious weaponry to defend our fatherland from Brest to Vladivostok,” Mr Lukashenko said, putting Belarus and its ally Russia under one umbrella.
In particular, he asked for help to make Belarus' military aircraft nuclear-capable.
Tensions between Russia and the West have soared since Moscow sent troops into Ukraine four months ago. Russia said that Nato planned to admit Ukraine and use it as a platform to threaten Moscow.
Russia's move has led to western sanctions and prompted Sweden and Finland to apply to join the alliance.
In the past week, Lithuania in particular has angered Russia by blocking movement of goods subject to European sanctions across its territory from Russia, through Belarus, to Russia's Baltic exclave of Kaliningrad.
Russia has termed that a blockade, but Lithuania said it affects 1 per cent of the normal goods in transit on the route, and that passenger traffic is unaffected.
Meanwhile, Russia attacked the Ukrainian capital in the early hours of Sunday morning, striking at least two residential buildings, Kyiv Mayor Vitali Klitschko said, as elsewhere Russian troops consolidated their gains in the east.
AP journalists in Kyiv said they saw rescue services battling fires and rescuing civilians.
Mr Klitschko said four people were taken to hospital with injuries and a seven-year-old girl was pulled alive from rubble.
Culture Minister Oleksandr Tkachenko said on the Telegram messaging app that a kindergarten was hit.
Ukrainian MP Oleksiy Goncharenko wrote on Telegram that “according to prelim data 14 missiles were launched against Kyiv region and Kyiv”.
Air Force spokesman Yuriy Ignat said the missiles were Kh-101 air-launched cruise missiles fired from over the Caspian Sea.
Before Sunday’s early morning attack, Kyiv had not faced any such Russian air strikes since June 5.
Mr Klitschko told journalists that he believed “it is maybe a symbolic attack” before the Nato summit in Madrid that starts on Tuesday.
Two more explosions were later heard in Kyiv, but their cause and possible casualties were not immediately clear.
Meanwhile, Russian forces have been seeking to swallow up the last remaining Ukrainian stronghold in the eastern Luhansk region, pressing their momentum after taking full control on Saturday of the city of Severodonetsk and the chemical plant where hundreds of Ukrainian troops and civilians had hidden.
Sergiy Gaiday, Governor of the Luhansk region that includes Severodonetsk, said on Sunday that Russia was conducting heavy air strikes on the adjacent city of Lysychansk, destroying its television tower and damaging a road bridge.
“There’s very much destruction — Lysychansk is almost unrecognisable,” Mr Gaiday wrote on Facebook.