Washington says it expects Russia to intensify a new offensive in Ukraine but expressed confidence in Kyiv and doubted that Moscow will make major territorial gains.
"It is possible that Russia will make further advances in the coming weeks, but we do not anticipate any major breakthroughs, and over time, the influx of U.S. assistance will enable Ukraine to withstand these attacks over the course of 2024," National Security Council spokesman John Kirby told reporters in Washington on May 10.
His comments came hours after Russian troops attempted to open a new front by breaking through Ukrainian lines in the Kharkiv region, a move Kyiv said its forces repelled, though fighting was reported to be continuing.
Kirby also spoke after the White House announced that the United States is preparing a $400 million military aid package for Ukraine, as the country returns to a regular pace of supplying weapons to Kyiv after lawmakers passed a bill that includes $61 billion in assistance for Ukraine.
Earlier in Kyiv at a press conference, President Volodymyr Zelenskiy said that the attack early in the morning on May 10 failed to catch Ukrainian troops by surprise.
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"Ukraine met them there with troops: brigades and artillery," Zelenskiy said, warning that Moscow could send more forces to back its attempted push that military sources said was aimed at establishing a "buffer zone" to prevent Ukrainian forces from shelling Russian territory.
The northeastern Ukrainian region of Kharkiv borders Russia, with the outskirts of its homonymous capital, Kharkiv, only 25 kilometers from the frontier.
Russian troops attacked after hours of heavy artillery fire, the press service of Ukraine's Defense Ministry reported.
"After dark, Russian occupiers stepped up the fire pressure on our defense line with the support of artillery. At approximately 5 a.m., the enemy attempted to break through our defense line under the cover of armored vehicles," the ministry said.
Reserve units have been deployed to strengthen the defense lines, and Ukrainian forces continue to hold back the enemy's offensive, the ministry said, adding that clashes of various intensity are still under way.
Earlier, Oleh Synyehubov, the governor of the Kharkiv region, reported that the town of Vovchansk, some 70 kilometers northeast of Kharkiv and only four kilometers from the Russian border, was coming under heavy Russian shelling.
"All night long, shelling from antiaircraft guns and artillery continued. There were unsuccessful [Russian] attempts...to break through the border," Synyehubov wrote on Telegram.
The Ukrainian Defense Ministry said fighting against Russian sabotage and reconnaissance groups had continued into the afternoon.
With a prewar population of 1.4 million, Kharkiv is Ukraine's second-largest city and one of its most important economic and industrial powerhouses and was one of the initial targets of Russia's invasion, experiencing heavy fighting in the spring of 2022.
A successful Ukrainian counteroffensive in September 2022 liberated most of the Kharkiv region, throwing Russian forces back and allowing the return of a large part of the city's population.
But Russian artillery, drone, and missile strikes on the region and its capital have intensified massively over the past several months as the Ukrainian forces' shortage of ammunition and air defense systems became more and more acute while a desperately needed U.S. military aid package was being held up by political bickering in the House of Representatives.
Many civilians have been killed and infrastructure has been seriously damaged in the region. A day before the May 10 Russian attacks, 21 settlements were hit by Russian artillery and mortar strikes, authorities reported.
Synyehubov said one person had been killed and at least three people had been wounded in that series of attacks.
Elsewhere in Ukraine, three civilians were killed.
One woman was killed in a Russian strike on Sumy region, local authorities reported.
Dnipropetrovsk Governor Serhiy Lysak said two people were killed in Nikopol and eight were wounded. In Donetsk, one person was wounded, regional head Vadym Filashkin said. Oleksandr Prokudin, the governor of Kherson, said one person was wounded in his region.
Separately, an oil refinery caught fire in Russia's Kaluga region after an alleged Ukrainian drone strike.
Kaluga Governor Vladislav Shapsha said the fire has been quickly localized and put out.
Ukraine has not commented on the claim, which could not be independently confirmed.
The Russian Defense Ministry said air defenses downed seven Ukrainian drones early on May 10 in the Moscow, Bryansk, and Belgorod regions.
Late on May 10, a suspected Ukrainian strike is reported to have triggered a large fire at an oil storage depot in Ukraine's occupied Luhansk region.
Russia-installed officials in the region said three people had been killed and seven wounded.