Russian Foreign Minister Sergei Lavrov gave Ukraine an ultimatum on Monday to fulfil Moscow's proposals, including surrendering territory Russia controls, or its army would decide the issue, a day after President Vladimir Putin said he was open to talks.
Kyiv and its Western allies have dismissed Putin's offer to talk, with his forces battering Ukrainian towns with missiles and rockets and Moscow continuing to demand that Kyiv recognize its conquest of a fifth of the country.
Kyiv says it will fight until Russia withdraws.
"Our proposals for the demilitarization and denazification of the territories controlled by the regime, the elimination of threats to Russia's security emanating from there, including our new lands, are well known to the enemy," state news agency TASS quoted Lavrov as saying late on Monday.
"The point is simple: Fulfil them for your own good. Otherwise, the issue will be decided by the Russian army."
Putin launched his invasion of Ukraine on Feb. 24, calling it a "special operation" to "denazify" and demilitarize Ukraine, which he said was a threat to Russia. Kyiv and the West say Putin's invasion was merely an imperialist land grab.
While Moscow had planned a swift operation to take over its neighbor, the war is now in its 11th month, marked by many embarrassing Russian battlefield setbacks and Ukraine’s successful defense of most of its land.
In the latest attack to expose gaps in Russia's air defenses, a drone believed to be Ukrainian penetrated hundreds of kilometers through Russian airspace on Monday, causing a deadly explosion at the main base for its strategic bombers.
Fierce fighting
Russian forces have been engaged for months in fierce fighting in the east and south of Ukraine, to defend the lands Moscow proclaimed it annexed in September and which make up the broader Ukrainian industrial Donbas region.
The Ukrainian top military command said on Monday that Russian forces carried out 19 attacks over the past day in the area. Russia's defense ministry said it had advanced its positions in the region and its missile troops and artillery had hit 63 Ukrainian units in the previous day.
In his nightly video message on Monday, Ukraine's President Volodymyr Zelenskiy called the situation along the frontline in Donbas "difficult and painful".
"Bakhmut, Kreminna and other areas in Donbas ... require a maximum of strength and concentration," Zelenskiy said.
"The occupiers are deploying all resources available to them - and these are considerable resources - to make some sort of advance."
Oleh Zhdanov, a military analyst based in Kyiv, said heavy fighting was going on around elevated areas near Kreminna in the Luhansk region.
He also said that fighting has picked up along the Bakhmut and Avdiivka, a line of contact further south in the Donetsk region, after a brief easing in previous days, with Russian forces launching a series of attacks in the area.
"The arc of fire in Donetsk region continues to burn. There has been very little change on either side of the front line in Donetsk region," Zhdanov said in a social media video post.
Zelenskiy said as a result of Russia's targeting of energy infrastructure nearly nine million people were without power. That figure amounts to about a quarter of Ukraine's population.
Sergey Kovalenko, head of YASNO, which supplies electricity to Kyiv, said late on Monday that while the power situation has been improving in the city, blackouts will continue.
"While repairs are underway, emergency shutdowns will continue," Kovalenko said on his Facebook page.
Tens of thousands of Ukrainian civilians have died in cities Russia razed to the ground, and thousands of troops on both sides have been killed, forcing Putin to call up hundreds of thousands of reservists for the first time since World War Two.
Russian airspace
Moscow on Monday said it had shot down a drone believed to be Ukrainian, causing it to crash at the Engels air base, where three service members were killed. Ukraine did not comment, under its usual policy on incidents inside Russia.
A suspected drone struck the same base on Dec. 5.
The base, the main airfield for the bombers that Kyiv says Moscow has used to attack Ukrainian civilian infrastructure, is hundreds of miles from the Ukrainian frontier. The same planes are also designed to launch nuclear-capable missiles as part of Russia's long-term strategic deterrent.
The Russian defense ministry said in a statement no planes were damaged, but Russian and Ukrainian social media accounts said several had been destroyed. Reuters was not able to independently verify the reports.
Putin hosted leaders of other former Soviet states in St. Petersburg on Monday for a summit of the Commonwealth of Independent States group, which Ukraine has long since quit.
The invasion of Ukraine has been a test of Russia's longstanding authority among other ex-Soviet states.
In televised remarks, Putin made no direct reference to the war, while saying threats to the security and stability of the Eurasian region were increasing.
"Unfortunately challenges and threats in this area, especially from the outside, are only growing each year," he said.
"We also have to acknowledge unfortunately that disagreements also arise between member states of the commonwealth."
