Russian President Vladimir Putin said on Monday that the Ukraine army’s incursion into the Kursk region, which has caused more than 100,000 civilians to flee, is an attempt by Kyiv to stop Moscow’s offensive in eastern Ukraine’s Donbas region.

Speaking at a meeting with top security and defense officials, Putin said the attack that began last Tuesday appeared to reflect Kyiv’s attempt to gain a better negotiating position in possible future talks to end the war.

He argued that Ukraine may have hoped to destabilize the domestic situation in Russia, adding that it has failed to achieve the goal, and claimed that the number of volunteers to join the Russian military has increased. He said the Russian military is driving on with its eastern Ukraine offensive.

Russia has seen previous incursions into its territory during the nearly 2 1/2-year war, but the foray into the Kursk region marked the largest attack on its soil since World War II, embarrassing President Vladimir Putin and constituting a milestone in the hostilities. It is also the first time the Ukrainian army has spearheaded an incursion rather than pro-Ukraine Russian fighters.

The combat inside Russia rekindled questions about whether Ukraine was using weaponry supplied by NATO members. Some Western countries have balked at allowing Ukraine to use their military aid to hit Russian soil, fearing it would fuel an escalation that might drag Russia and NATO into war.

Though it’s not clear what weapons Ukraine is using across the border, Russian media widely reported that US Bradley and German Marder armoured infantry vehicles were there. It was not possible to independently verify that claim.

Ukraine has already used US weapons to strike inside Russia.

But Italian Foreign Minister Antonio Tajani said in an interview published Monday that the weapons provided by his country “cannot be used to attack Russia on its territory”.

Meanwhile, German Defence Ministry spokesperson Arne Collatz said on Monday that legal experts agree that “international law provides for a state that is defending itself also to defend itself on the territory of the attacker. That is clear from our point of view, too”.

He said that the criterion for the use of weapons, at least where those supplied from German military stocks is concerned, is that Ukraine only uses them within the limits of international law.

Russian air force and artillery also struck concentrations of Ukrainian troops and equipment near Sudzha, Kurilovka, Pekhovo, Lyubimovo and several other settlements, it said. Warplanes and artillery also hit Kyiv’s reserves in Ukraine’s Sumy region across the border, it said.

Pasi Paroinen, an analyst with the Finland-based Black Bird Group open-source intelligence agency, which monitors the war, said the toughest phase of Ukraine’s incursion is likely to begin now as Russian reserves enter the fray.

He said that “if the Ukrainians are going to advance any further from where they are now, it’s going to be a tough battle, unlike the opening moments of this offensive”.

Ukraine’s progress on Russian territory “is challenging the operational and strategic assumptions” of the Kremlin’s forces, according to the Institute for the Study of War.

It could compel Russia to deploy more military assets to the long border between the two countries, the Washington-based think tank said in an assessment late Sunday.

It described the Russian forces responding to the incursion as “hastily assembled and disparate”.