Zelenskyy acknowledges push into Russia to put 'pressure on the aggressor'

LONDON -- Ukraine is continuing to try to expand its surprise incursion into Russia's Kursk region, with Russia's military reporting multiple attempts Sunday night by Ukrainian forces to push deeper into the region, as President Volodymyr Zelenskyy for the first time confirmed the operation was underway since it began six days ago.

The incursion, which some military analysts have assessed likely involves up to several thousand Ukrainian troops, has stunned Moscow and sent 76,000 Russian civilians fleeing from the border area, according to Russian authorities.

Ukrainian forces, using Western-supplied tanks and armored vehicles, have seized several villages and managed to advance as deep as about 12 miles during the operation, which is the first major foreign military invasion of Russia since World War II.

Russia has been racing to move reinforcements to the region to reverse the incursion, but has so far been unable to push the Ukrainians out. Ukraine's forces have been seeking to expand their bridgehead around the town of Sudzha, close to the border.

Ukraine's attack began last week and appeared to be a large-scale offensive operation, involving at least two Ukrainian brigades.

Ukrainian officials have been closed-lipped about the operation, but Zelenskyy late Saturday broke his silence on it. In an evening address, Zelenskyy said his commander-in-chief Oleksandr Syrskiy had briefed him several times on Ukraine's "actions to push the war out into the aggressor's territory."

Zelenskyy did not mention Kursk directly but thanked Ukraine's units, adding: "Ukraine is proving that it really knows how to restore justice and guarantees exactly the kind of pressure that is needed -- pressure on the aggressor."

Ukraine is reportedly still moving more troops into the Kursk region, according to Russian accounts and journalists on the border in Ukraine's northeastern Sumy region.

Russia's Defense Ministry on Sunday said it had stopped Ukrainian forces from advancing at three villages located roughly 18 miles inside the Kursk region. In its briefing, it wrote that Russian forces, using aviation, drones and artillery, had blocked "attempts to break through deep into Russian territory" by "mobile groups with armored vehicles" close to the villages of Tolpino, Zhuravli and Obshchy Kolodez.

Ukraine has not significantly advanced since the first two days of the incursion, according to independent open-source military analysts who track the conflict, suggesting Russia may be beginning to stabilize its position. But Russian forces were still struggling to contain the Ukrainian push, according to Russian pro-Kremlin military bloggers, who have warned that Ukraine may also seek to open new fronts elsewhere on the border.

Some military analysts say the combat area in Kursk is now estimated to cover more than 350 square kilometers.

Russia's Defense Ministry has claimed Ukrainian forces are suffering heavy casualties, claiming Ukraine has lost more than 1,100 service members and more than 100 armored vehicles, including 22 tanks, since the incursion began.

More Russian conscripts were moved into the region in recent days, including some who had been redeployed from front-line positions elsewhere, according to the Institute for the Study of War, a nonprofit think tank in Washington, D.C.

"Russian forces appear to be more adequately defending against Ukrainian assaults following the arrival of additional conscripts and more combat-effective personnel from front-line areas in Ukraine," the think tank said Saturday.

Small Ukrainian units have been spotted in villages northeast toward the Kursk nuclear power station, which is located roughly 80 miles from the border, according to accounts from Russian military bloggers. Russia has beefed up security at the plant, though most analysts believe it is still too far for Ukrainian troops to reach.

Some military analysts have warned that Ukraine's operation using elements from some of its most elite brigades is a risky gamble, given its forces are stretched thin elsewhere.

Ukraine's ultimate objective for the operation is still unclear. Most analysts believe it likely seeks to pull Russian forces from elsewhere in Ukraine where Ukrainian forces are under pressure. Some analysts suggest it also attempts to shift the perception of the war, which has seen Ukraine recently facing a seemingly inexorable Russian advance in eastern Ukraine.

The move could also be aimed at strengthening Ukraine's position in any future negotiations, analysts say, including perhaps even seizing territory to eventually trade for occupied Ukrainian land.

A prominent Russian pro-Kremlin military blogger, Yury Kotenok, on Sunday suggested on his Telegram account that Russian forces were being pulled from areas where Russia has been successfully on the offensive close to the strategic Ukrainian city of Pokrovsk in Donbas to be sent to Kursk.

ABC News' Patrick Reevell and Kevin Shalvey contributed to this report.