U.S. Secretary of State Antony Blinken said on Tuesday that the American military aid heading to Ukraine will make a "real difference" on the battlefield, as the senior diplomat made an unannounced visit to Kyiv to reassure the ally facing a fierce new Russian offensive.

In increasingly intense attacks along the northeastern border in recent days, Russian forces have seized about 100 to 125 square kilometers (40 to 50 square miles) in the Kharkiv region, which includes at least seven villages, according to monitoring analysts. Although most of those villages were already largely abandoned, thousands of civilians in the area have fled the fighting.

The Kremlin's forces are also conducting a coordinated assault in the east, seeking to penetrate deeper into the partially occupied Donetsk region. The Ukrainian General Staff reported that Russian attacks were mainly focused on Pokrovsk on Tuesday, directly within the Ukrainian borders in Donetsk, where Kremlin forces launched 24 attacks.

Analysts described this moment as one of the most dangerous times for Ukraine since the large-scale Russian invasion in February 2022, and President Volodymyr Zelenskyy on Tuesday requested more air defense systems to protect civilians under Russian fire in the northeast of the country.

Speaking in the Ukrainian capital where he met with Zelenskyy, Blinken said: "We know this is a time filled with challenges." He added that U.S. military aid "will make a real difference against the ongoing Russian aggression on the battlefield."

The visit comes less than a month after Congress approved a long-awaited foreign aid package that allocated $60 billion in assistance to Ukraine, most of which will go towards replenishing severely depleted artillery and air defense systems.

Blinken stated that some of that "is on the way now," and some has already arrived in Ukraine.

The renewed Moscow offensive in the northeastern Kharkiv area is the most significant border incursion since the early days of the war and comes after months when the front line, approximately 1,000 kilometers (620 miles) long, had barely shifted.

More than 7,500 civilians have been evacuated from the area, according to authorities. Meanwhile, the Kremlin's forces are expanding their advance into the northern border regions of Sumy and Chernihiv, Ukrainian officials say, as outnumbered and outgunned Kyiv soldiers struggle to repel them.

Oleh Syniehubov, the regional governor, said on national television that troops were engaged in street-to-street fighting on the outskirts of Vovchansk, one of the largest cities in the Kharkiv region. He added that civilians were killed in Russian shelling on Tuesday.

The United Nations Human Rights Office reported that the fighting has resulted in heavy casualties.

Liz Throssell, spokesperson for the UN High Commissioner for Human Rights, said in Geneva: "We are deeply concerned about the plight of civilians in Ukraine. In the Kharkiv region, the situation is dire."

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

"People are being attacked: civilians, fighters, everyone. They are under Russian missiles."

A senior U.S. official traveling with the secretary on an overnight train from Poland, speaking to reporters on condition of anonymity before talks with Blinken, said artillery, interceptor air defense missiles, and long-range ballistic missiles have already been delivered, with some already reaching the front lines.

Before the trip, U.S. officials indicated that since President Joe 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 long-term support.

During his fourth trip to Kyiv since Russian forces crossed the border, Blinken told Ukrainian Prime Minister Denys Shmyhal that the United States intends to support Kyiv after the war's end.

Blinken affirmed: "The United States is determined and committed to helping Ukraine succeed, to succeed in victory on the battlefield, but also to succeed, as we say, in winning peace and building the strongest possible Ukraine."

National Security Advisor Jake Sullivan stated that the administration "is already trying to accelerate the pace" of U.S. arms shipments.

However, delays in U.S. aid, especially since the war broke out between Israel and Hamas, which has occupied senior administration officials, have raised deep concerns in Kyiv and Europe. For example, Blinken visited the Middle East seven times since the Gaza war began in October. His last trip to Kyiv was in September.

Blinken joined Ukrainian Foreign Minister Dmytro Kuleba for lunch at a pizza restaurant in Kyiv founded by Ukrainian veterans, describing it as "fantastic." On Blinken's last visit, the two dined at a recently reopened McDonald's.

The Secretary of State is scheduled to deliver a speech later on Tuesday praising the "strategic successes" achieved by Ukraine in the war. This speech aims to complement Blinken's address last year in Helsinki, Finland, where he mocked Putin for Moscow's strategic failures in waging the war.

However, since the Helsinki speech, Russia has intensified its attacks, most notably as the U.S. House of Representatives has been considering the aid package for months without taking action, which has led to the suspension of most American assistance. Those attacks have increased in recent weeks as Russia sought to exploit Ukraine's shortages in manpower and weapons while waiting for new assistance.

Meanwhile, Russian President Vladimir Putin is set to make a two-day international visit to China this week, according to the Chinese Foreign Ministry. Beijing has politically supported Moscow in the war and has sent it mechanical and electronic tools and other equipment seen as contributing to the Russian war effort without actually exporting weapons.

Blinken told Zelenskyy in Kyiv: "A strong, successful, prosperous, and free Ukraine is the best possible rebuke to Putin and the best possible guarantee for your future."

The senior U.S. official noted that despite some recent setbacks, Ukraine can still achieve significant victories. This includes regaining about 50% of the territory seized by Russian forces in the early months of the war, improving its economic standing, and enhancing transportation and trade links, particularly through military successes in the Black Sea.