A Russian missile attack on a large Ukrainian military facility near the border with NATO member Poland on Sunday killed 35 people and wounded 134, a Ukrainian official said.
Britain said the incident, just 15 miles (25 km) from the Polish border, marked a "significant escalation" of the conflict. US President Joe Biden has said NATO will defend every inch of its territory if Russia's invasion of Ukraine spills over into member states of the Western defence alliance.
Ukraine said foreign military instructors have previously worked at the Yavoriv International Centre for Peacekeeping and Security, but a NATO official said there were no personnel from the alliance there. It was not immediately clear whether any non-NATO states might have representatives there.
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Regional governor Maksym Kozytskyy said Russian planes fired around 30 rockets at facility, adding that some were intercepted before they hit. At least 35 people were killed and 134 wounded, he said. Reuters was not able to verify his statement.
The 360 square-km (140 square-mile) facility is one of Ukraine's biggest and the largest in the western part of the country.
The Kremlin did not immediately reply to a request for comment on the reported missile strike so close to the border with NATO, and a short video briefing by Russia's defence ministry spokesman on Sunday made no mention of such an attack.
Nineteen ambulances with sirens on were seen by Reuters driving from the direction of the Yaroviv facility after the reported strike and black smoke rose from the area.
"Russia has attacked the International Centre for Peacekeeping & Security near Lviv. Foreign instructors work here. Information about the victims is being clarified," Ukrainian Defence Minister Oleksii Reznikov said in an online post.
A Ukrainian defence ministry representative told Reuters the ministry was still trying to establish if any foreign instructors were at the centre at the time of the attack.
Ukraine, whose aspirations to join NATO are a major irritant to Russian President Vladimir Putin, held most of its drills with countries in the Western defence alliance at the base before the invasion. The last major exercises were in September.
In the weeks before Russia invaded on Feb. 24, the Ukrainian military-trained there, but according to Ukrainian media, all foreign instructors left the training ground in mid-February, while leaving all the equipment.
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Russia's invasion of Ukraine, launched by Putin on Feb. 24, has sent more than 2.5 million fleeing across borders and trapped thousands of people in besieged cities.
While Western nations have sought to isolate Putin by imposing harsh sanctions, the United States and its allies are concerned to avoid NATO being drawn into the conflict. "There are no NATO personnel in Ukraine," the NATO official said, when asked if any NATO personnel were at the base.
Asked how much of an escalation the attack was, UK cabinet minister Michael Gove told a BBC interviewer: "It is significant." Putin was pushing the boundaries in military terms, he said.
Airport hit in West
The mayor of another city in western Ukraine, Ivano-Frankivsk, said Russian troops also continued to hit its airport, with no initial reports of casualties.
In eastern Ukraine, Russian troops are trying to surround Ukrainian forces as they advance from the port of Mariupol in the south and the second city Kharkiv in the north, the UK Defence Ministry said on Sunday.
Kharkhiv has suffered some of the heaviest bombardment. Videos from one resident, Teimur Aliev, who is helping bring aid to residents, show bombed buildings lining streets, burned out cars riddled with shrapnel holes and debris strewn around.
"We will stitch up the wounds and the pain of our country and our city. We are ready to build it and we are ready to renew it when the war is over. We're not going anywhere," Aliev, a 23-year-old musician, said of his food distribution network, which now has dozens of volunteers.
British intelligence also said Russian forces advancing from Crimea, which Russia annexed from Ukraine in 2014, were trying to circumvent Mykolaiv as they look to drive west towards Odesa. Air strikes on Mykolayiv killed nine people on Sunday, regional Governor Vitaliy Kim said in an online statement.
Air raid sirens again woke residents in Kyiv on Sunday morning, hours after Ukrainian President Volodymyr Zelensky warned Russian forces they face a fight to the death if they try to occupy the capital.
Ukraine accused Russian forces on Saturday of killing seven civilians, including one child, in an attack on women and children trying to flee fighting near Kyiv.
Reuters was unable immediately to verify the report and Russia offered no immediate comment.
Moscow denies targeting civilians. It blames Ukraine for failed attempts to evacuate civilians from encircled cities, an accusation Ukraine and its Western allies strongly reject.
More arms for Ukraine
The United States said it would rush up to $200 million in additional small arms, anti-tank and anti-aircraft weapons to Ukraine, where officials have pleaded for more military aid.
The Kremlin describes its actions as a "special operation" to demilitarise and "deNazify" Ukraine. Ukraine and Western allies call this a baseless pretext for a war of choice.
Polish President Andrzej Duda warned in an interview broadcast on Sunday that the use of chemical weapons in Ukraine by Russia would be a game changer and NATO would have to think seriously about how to respond.
Russian troops have destroyed 3,687 Ukrainian military infrastructure facilities so far, Russian Defence Ministry spokesman Igor Konashenkov was quoted by Russian news agencies as saying on Sunday. It was not possible to independently verify his statement.