Kyiv, Ukraine — U.S. Secretary of State Antony Blinken said Tuesday that American military aid on its way to Ukraine will make a “real difference” on the battlefield, as the top diplomat made an unannounced visit to reassure an ally facing a fierce new Russian offensive. In increasingly intense attacks along Ukraine’s northeast border in recent days, Moscow’s troops have captured around 40 to 50 square miles of territory, including at least seven villages, according to open source monitoring analysts.

Though most of those villages were already depopulated, thousands of civilians in the area have fled the fighting, and analysts have called it one of the most dangerous moments for Ukraine since Russia launched its full-scale invasion in February 2022.

President Volodymyr Zelenskyy renewed his longstanding, but possibly more urgent request for more air defense systems to protect civilians under Russian fire in the northeast as he met Blinken on Tuesday.

“We know this is a challenging time,” Blinken said in the Ukrainian capital, where he met with Zelenskyy. But he added that American military aid is “going to make a real difference against the ongoing Russian aggression on the battlefield.”

The visit comes less than a month after Congress approved a long-delayed foreign assistance package that sets aside $60 billion in aid for Ukraine, much of which will go toward replenishing badly depleted artillery and air defense systems.

Some of that “is now on the way,” Blinken said, and some has already arrived in Ukraine.

Moscow’s renewed offensive in the northeast region of Kharkiv is the most significant border incursion since the early days of the war — and comes after months when the roughly 600-mile front line barely budged. It was not, however, unexpected.

Zelenskyy told CBS News’ Charlie D’Agata at the end of March, as his country waited for U.S. politicians to clear the aid package, that while his troops had managed to keep the Russians largely at bay up to that point, they were not prepared to defend against another major Russian offensive, which he said was expected in the coming months.

The Ukrainian leader warned in the interview that if his country’s Western partners failed to help stop Russia’s invasion, “this aggression, and Putin’s army, can come to Europe, and then the citizens of the United States, the soldiers of the United States, will have to protect Europe, because they’re the NATO members.”

More than 7,500 civilians have been evacuated from the area, according to authorities. The Kremlin’s forces are also expanding their push to the northern border regions of Sumy and Chernihiv, Ukrainian officials say, and Kyiv’s outgunned and outnumbered soldiers are struggling to hold them back.

Troops fought street to street on the outskirts of Vovchansk, which is among the largest towns in the Kharkiv area, regional Gov. Oleh Syniehubov said on national television. Two civilians were killed in Russian shelling Tuesday, he said.

The U.N. human rights office said the battles are taking a heavy toll.

“We are deeply concerned at the plight of civilians in Ukraine,” Liz Throssell, spokeswoman for the office of the U.N. High Commissioner for Human Rights, said in Geneva. “In the Kharkiv region, the situation is dire.”

Zelenskyy thanked Blinken for the U.S. aid — but added that more is necessary, including two Patriot air defense systems that are urgently needed to protect Kharkiv.

“The people are under attack: civilians, warriors, everybody. They’re under Russian missiles,” he said.

Artillery, air defense interceptors and long-range ballistic missiles have already been delivered, some of them already to the front lines, said a senior U.S. official traveling with the secretary on an overnight train from Poland who spoke to reporters on condition of anonymity ahead of Blinken’s meetings.

Ahead of the trip, U.S. officials noted that since President Biden signed the aid package late last month, the administration has already announced $1.4 billion in short-term military assistance and $6 billion in longer-term support.

The administration is “trying to really accelerate the tempo” of U.S. weapon shipments, said national security adviser Jake Sullivan.

But delays in U.S. assistance, particularly since the Israel-Hamas war began and has preoccupied top administration officials, have triggered deep concern in Kyiv and Europe. Blinken, for example, has visited the Middle East seven times since the Gaza conflict began in October. His last trip to Kyiv was in September.

On his fourth trip to Kyiv since Russian troops rolled over the border, Blinken emphasized the United States’ support for Ukraine’s independence and eventual recovery, according to spokesperson Matthew Miller. The U.S. diplomat and the Ukrainian president also discussed long-term security arrangements and Ukraine’s economic welfare.

Blinken was due to give a speech later Tuesday extolling Ukraine’s “strategic successes” in the war. It is intended to complement a Blinken address last year in Helsinki, Finland, deriding Putin for Moscow’s strategic failures in launching the war.

Since the Helsinki speech, however, Russia has intensified its attacks, most noticeably as the U.S. House of Representatives sat on the aid package for months without action, forcing a suspension in the provision of most U.S. assistance. Those attacks have increased in recent weeks as Russia has sought to take advantage of Ukrainian shortages in manpower and weapons while the new assistance is in transit.

Meanwhile, Russian President Vladimir Putin plans to make a two-day state visit to China this week, the Chinese Foreign Ministry said. Beijing has backed Moscow politically in the war and has sent machine tools, electronics and other items seen as contributing to the Russian war effort, without actually exporting weaponry.

“A strong, successful, thriving, free Ukraine is the best possible rebuke to Putin and the best possible guarantor for your future,” Blinken told Zelenskyy in Kyiv.

The senior U.S. official said despite some recent setbacks, Ukraine could still claim significant victories. Those include reclaiming some 50% of the territory Russian forces took in the early months of the war, boosting its economic standing and improving transportation and trade links, not least through military successes in the Black Sea.

The official acknowledged that Ukraine faces “a tough fight” and is “under tremendous pressure” but argued that Ukrainians “will become increasingly more confident” as the new U.S. and other Western assistance begins to arrive.