Ukrainian President Volodymyr Zelenskiy has cited a "series of victories" on the battlefield against invading Russian forces following reports that Ukrainian troops reached the eastern bank of a major river in the northeast, but he joined Western military and other officials cautioning of new risks in the nearly seven-month-old conflict.
An attack by Russian troops on a nuclear power plant near Mykolayiv in the south reportedly did not damage any reactors but underscored ongoing risks to Ukrainian atomic facilities.
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Zelenskiy warned late on September 18 that any "lull" was part of "preparation for the next series" of operations. Meanwhile, President Joe Biden said that while Ukrainians "are defeating Russia," there's still much more to be done.
The chairman of the U.S. Joint Chiefs of Staff signaled concerns about how Moscow might react to setbacks in battle, urging U.S. troops in Europe and allies to be at "high states of readiness [and] alert."
"Maybe it seems to some of you that after a series of victories we now have a lull of sorts," Zelenskiy said in his nightly video address. "But there will be no lull. There is preparation for the next series.... For Ukraine must be free. All of it."
Earlier in the day, Ukraine's military said via Telegram that its troops had established a bridgehead ver the Oskil River in their counteroffensive in the northeastern Kharkiv region, where they appear to have retaken thousands of square kilometers so far this month.
Experts suggest the foothold on the Oskil could threaten Russian troops in the Donbas region of eastern Ukraine that Moscow has suggested is a priority and where Russia-backed separatists have held swaths of territory for eight years.
The Oskil flows south to the Siverskiy Donets River, which bisects the Luhansk region that along with Donetsk composes what's known as the Donbas.
Ukraine's military said late on September 18 that its troops "have pushed across the Oskil," adding, "From yesterday, Ukraine controls the east bank."
WATCH: The speed and efficiency of Ukraine's counteroffensive in the northeastern region of Kharkiv came as a stunning surprise to the Russian military. Ukraine went to great lengths to keep its counteroffensive secret, including deliberately deceiving Russian forces about its military maneuvers.
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How Russian Forces Were 'Deceived' In Ukraine's Kharkiv Counteroffensive
The governor of Luhansk, Serhiy Hayday, wrote on Telegram that "Luhansk region is right next door. Decoccupation is not far away."
Early on September 19, the Ukrainian General Staff said there had been six Russian missile strikes in the past day and a half, 22 air strikes, and more than 90 strikes from multiple-rocket launchers.
It said infrastructure in more than 30 cities and towns had been damaged and warned Ukrainians of the continued threat of air and missile strikes all over the country.
The Ukrainian nuclear operator, Enerhoatom, accused Russian troops of striking the Pivdennoukrayinska nuclear power plant in the southern Mykolayiv region but said none of its three reactors had been damaged. The facility is functioning normally, Enerhoatom said.
Ukrainian officials on September 18 accused Russian troops of pounding civilian infrastructure with artillery including in the city of Zaporizhzhya, where Europe's largest nuclear facility lies.
Russians have occupied Zaporizhzhya since shortly after the February launch of their invasion, and electricity was reportedly restored over the weekend to the plant to help allay fears of a catastrophic disaster there.
RFE/RL cannot independently verify battlefield claims in areas of the most intense fighting.
WATCH: The speed and efficiency of Ukraine's counteroffensive in the northeastern region of Kharkiv came as a stunning surprise to the Russian military. Ukraine went to great lengths to keep its counteroffensive secret, including deliberately deceiving Russian forces about its military maneuvers.
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Ukrainians Find Bodies With Hands Tied In Mass Burial Site Near Izyum
After a visit to a base in Poland where troops from the United States and other countries are aiding Ukraine's defenders, U.S. Army General Mark Milley, who is chairman of the Joint Chiefs of Staff, said "the war is not going too well for Russia right now."
While he said U.S. troops in Europe are not under immediate threat, questions about Russian President Vladimir Putin's response to battlefield losses mean it is "incumbent upon all of us to maintain high states of readiness [and] alert."
U.S. and NATO officials have warned Moscow that any attack on NATO territory would elicit a strong military response.
President Biden said during a television appearance that "winning the war in Ukraine is to get Russia out of Ukraine completely and to recognize [Ukrainian] sovereignty."
He suggested that "they're defeating Russia," and said, "Russia's turning out not to be as competent and capable as many people thought they were going to be."
Asked about the risk of Putin considering the use of nuclear weapons amid setbacks in its Ukraine war, Biden had simple words for the Russian president: "Don't. Don't. Don't. It would change the face of war unlike anything since World War II."