Russian President Vladimir Putin says Ukraine is trying to undermine Russian stability with its incursion into the south of the country and warns that Ukraine will receive “a worthy response”.

Last week, Ukrainian forces rammed through the Russian border and swept across western parts of the Kursk region. The incursion is Ukraine’s biggest across the border since the start of Russia’s war in the country in 2022.

Apparently caught by surprise, Moscow has hit back militarily by deploying its own troops to quash the incursion. Russia’s Defence Ministry said on Monday that additional forces and resources had arrived in Kursk, without elaborating on the numbers.

“Heavy tracked vehicles are being loaded onto automobile trailers for prompt delivery to the areas where Ukrainian Armed Forces formations are being blocked and to ensure the safety of the road surface,” the military reported.

On Monday, Ukraine’s army chief, Oleksandr Syrskyii, said Kyiv controls about 1,000sq km (386sq miles) of Kursk, according to a video excerpt of his report shared by President Volodymyr Zelenskyy on Telegram.

Monday was also the first time Zelenskyy confirmed that Ukrainian military forces are operating inside Kursk. In his nightly address the president said war was now coming back to Russia after Moscow had taken fighting to other countries.

Earlier in the day, Kursk Governor Alexei Smirnov told Putin that six days of Ukrainian ground attacks on his region had resulted in the loss of 28 settlements, and the incursion was about 12km deep and 40km wide.

He said 12 civilians had been killed and 121,000 people were evacuated or left the areas affected by fighting on their own. The total planned number of evacuations is 180,000.

False accusations?

On Monday, a new evacuation order was also issued in Russia’s Belgorod region as its Krasnaya Yaruga district feared an incursion by Ukrainian forces, according to Vyacheslav Gladkov, the regional governor.

Krasnaya Yaruga is located in the northwest of Belgorod and borders Kursk.

Putin said at a televised meeting: “The losses of the Ukrainian armed forces are increasing dramatically for them, including among the most combat-ready units, units that the enemy is transferring to our border.”

“The enemy will certainly receive a worthy response, and all the goals facing us will, without a doubt, be achieved,” the president added.

Ukraine’s Security Service (SBU) said Russia was trying to falsely accuse Kyiv’s military of war crimes.

The SBU said on Telegram that it had gathered information showing Russia may stage crimes that it would, in turn, blame on Ukraine.

Moscow and Kyiv regularly accuse each other of planning so-called false flag operations.

Military analyst Sean Bell told Al Jazeera that while critics deemed Ukraine’s surprise attack reckless, “Momentum and initiative is everything in warfare. This caught everybody off guard.”

Kyiv ready for retaliation?

Ukraine has also been bracing for more Russian attacks in retaliation for its recent cross-border incursion.

On Sunday, at least two people were killed and three injured in a Russian air attack outside Kyiv. Ukraine also announced it had evacuated 20,000 people from the Sumy region, which sits across the border from Kursk, as the fighting in the area intensified.

Moscow and Kyiv have also accused each other of starting a fire on the grounds of Europe’s largest and now Russian-occupied Zaporizhzhia Nuclear Power Station in Ukraine on Sunday with both sides reporting no sign of elevated radiation.

Zelenskyy, meanwhile, has reiterated his calls to Ukraine’s allies for “a full-fledged air shield that can protect all our cities and communities”.

“Ukrainians see a prospect of a long war, a difficult war, a bloody war,” Lieutenant General Ihor Romanenko, former deputy head of the General Staff of the Ukrainian Armed Forces, told Al Jazeera.

Ukraine needs two things – better arms and ammunition supplies and a massive nationwide mobilisation that exceeds highly unpopular recent steps to enlist tens of thousands of men, he added.

Meanwhile, China’s Ministry of Foreign Affairs has urged Russia and Ukraine to follow “three principles for de-escalating the situation”.

There should be “no expansion of the battlefield, no escalation of fighting and no fuelling the flame by any party”, a ministry spokesperson said in a statement, adding that China will “play a constructive role in promoting a political solution to the crisis”.

China presents itself as a neutral party in the war and says it is not sending lethal assistance to either side, unlike the United States and other Western nations. But it is also a close political and economic ally of Russia, and NATO members have branded Beijing a “decisive enabler” of the war, which it has never condemned.