KYIV, Ukraine — A major Russian drone and missile attack on civilian areas of Ukraine killed four people and injured at least 27, officials said Tuesday, while Moscow’s army stepped up efforts to break through Ukrainian front-line defenses in what could be the start of an anticipated spring ground offensive.

Russia fired almost 400 long-range drones at Ukraine overnight, Ukraine’s air force said, in its biggest attack in weeks. The onslaught continued into Tuesday morning as dozens of drones targeted the capital Kyiv during daylight.

Russia also launched 23 cruise missiles and seven ballistic missiles at Ukraine during the night, hitting at least 10 locations across the country, according to the air force.

Ukrainian civilians have endured relentless barrages since Russia launched a full-scale invasion of its neighbor more than four years ago. U.S-brokered talks between Moscow and Kyiv over the past year have brought no respite, with Russia rejecting Ukraine’s offer of a ceasefire, and in recent weeks the Iran war has diverted international attention from Ukraine’s plight.

On the roughly 1,250-kilometer (750-mile) front line snaking along eastern and southern parts of Ukraine, the short-handed defenders have been bracing for a new offensive by Russia’s bigger army as the weather improves.

The Commander-in-Chief of Ukraine’s armed forces, Gen. Oleksandr Syrskyi, said Russian troops in recent days have made simultaneous attempts to break through defensive lines in several strategic areas.

“Fierce fighting unfolded along the entire line of contact,” Syrskyi said Monday on the Telegram messaging app, with Russia launching 619 attacks in four days.

“The occupiers are attempting to bring up new units and are preparing to continue attacks,” Syrskyi said, adding that Ukraine had deployed reinforcements to counter the assaults.

The Institute for the Study of War, a Washington-based think tank, said Syrskyi’s report backed up its assessment that Russia’s spring-summer offensive is now underway.

Russia has escalated its strikes since March 17 and has moved heavy equipment and more troops to the front line, the ISW said late Monday.

Each year, as the weather improves, Russia has moved its grinding war of attrition up a gear. However, it has been unable to capture cities and has made only incremental gains across rural areas. Russia occupies about 20% of Ukraine. That includes the Crimean Peninsula, which Russia seized in 2014.

Ukraine has developed advanced drone technology to make up for its shortage of infantry.

Amid the Middle East conflict, Kyiv is offering Ukraine’s battle-tested drone defenses to U.S. and Gulf partners, hoping to trade that know-how for scarce Patriot air defense missiles it needs to fend off Russia’s barrages.

Ukraine has also used its domestically produced long-range drones to hit areas of Russia that support Moscow’s war effort. Russian air defenses intercepted 55 Ukrainian drones overnight over Russian regions, the annexed Crimea and the Black Sea, the Russian Defense Ministry reported Tuesday.

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