By Guy Faulconbridge and Lidia Kelly
MOSCOW (Reuters) -Russia said on Wednesday it was fighting intense battles against Ukrainian forces that had penetrated its southern border near a major natural gas transmission hub, in one of the largest incursions into Russian territory since the war began.
The acting governor of Kursk region, Alexey Smirnov, said he had introduced a state of emergency in the border region. Regional officials said that meant restricting access to specific areas.
Russia's health ministry said 31 civilians, including six children, had been wounded. Smirnov said on Tuesday that five people had been killed.
No information on military casualties was available.
Russia's National Guard said it had beefed up security around the nearby Kursk nuclear power station and its four reactors.
Russia has advanced this year after the failure of Ukraine's 2023 counteroffensive to achieve major gains, and has taken 420 square km (162 square miles) of territory from Ukrainian forces since June 14, Russian officials say.
Ukraine struck back on Tuesday, and battles continued through the night into Wednesday as Ukrainian forces pushed to the northwest of the border town of Sudzha, 530 km (330 miles) southwest of Moscow, Russia's defence ministry said.
"The Kyiv regime has launched another major provocation," President Vladimir Putin told members of the Russian government, referring to the attack in Kursk region.
NEARLY 1,000 UKRAINIAN SOLDIERS
The chief of Russia's general staff, Valery Gerasimov, told Putin that Russian forces had halted a thrust by up to 1,000 Ukrainian soldiers - more than three times the figure that Russia's defence ministry had stated on Tuesday - and would push them back to the border.
The advance was stopped by "the actions of the units covering the state border together with border guards and reinforcement units, with airstrikes, missile and artillery fire," Gerasimov said in televised comments.
The Ukrainian military appeared to have adopted a strategy of strict silence.
President Volodymyr Zelenskiy, in his nightly video address on Wednesday, made no reference to the attack, while exhorting Kyiv's soldiers to press on and weaken Russian forces.
Ukraine's General Staff also made no acknowledgment in its daily battlefield update. In a late evening report, it said fighting had intensified in Sumy region -- across the border from Russia's Kursk region.
The General Staff said Russian forces had deployed aircraft, helicopters and heavy weapons in the area "but made no headway and suffered significant losses".
'NO VIOLATION' OF RULES ON WEAPONS, WASHINGTON SAYS
In Washington, White House spokesperson Karine Jean-Pierre said the United States was seeking an understanding from Ukraine of the incursion, and said it had had no advance knowledge of it.
State Department spokesperson Matthew Miller said U.S. rules on Ukrainian use of U.S. weapons - authorised in areas over the Russian border - remained in effect, but that Ukraine's actions were "not a violation of our policy."
The Russian ambassador to Washington, Anatoly Antonov, said on Telegram that U.S. statements on the Ukrainian action were "outrageous...not a word criticising their clients, not a regret about the victims of the tragedy".
There was fighting around Sudzha, the last operational trans-shipping point for Russian natural gas to Europe via Ukraine. The Urengoy–Pomary–Uzhhorod pipeline carried about 14.65 billion cubic metres of gas in 2023, about half of Russia's gas exports to Europe.
Ukraine's gas transmission operator said Russian gas was transiting to European consumers normally. Just 60 km to the northeast lies the Kursk nuclear power station.
The battles around Sudzha come at a crucial juncture in the conflict, the biggest land war in Europe since World War Two. Kyiv is concerned that U.S. support could drop off if Republican Donald Trump wins the November presidential election.
Trump has said he would end the war, and both Russia and Ukraine are keen to gain the strongest possible bargaining position on the battlefield. Ukraine wants to pin down Russian forces and show the West it can still mount major battles.
Russian military bloggers depicted the situation in Kursk region as more serious than the official accounts, with some suggesting that Ukraine had opened a new front. Russia has sent reserves to help shore up its defences.
Some bloggers suggested that Ukraine might be planning an advance on the Kursk nuclear plant.
Both Kyiv and Moscow say they do not target civilians in the war, triggered by Russia's full-scale invasion in February 2022.
Russian Telegram channels carried unverified footage of shelled houses.
Forces describing themselves as voluntary paramilitaries fighting on Ukraine's side penetrated parts of Kursk and the adjacent Belgorod region earlier this year, triggering a push by Russian troops to set up a buffer zone in Ukraine's northeast.