U.S. Secretary of State Antony Blinken is greeted by U.S. Ambassador to Ukraine Bridget A. Brink after arriving by train at Kyiv-Pasazhyrskyi station on Tuesday in Kyiv, Ukraine.

U.S. Secretary of State Antony Blinken is greeted by U.S. Ambassador to Ukraine Bridget A. Brink after arriving by train at Kyiv-Pasazhyrskyi station on Tuesday in Kyiv, Ukraine. / AP

KYIV, Ukraine — U.S. Secretary of State Antony Blinken arrived in Kyiv on Tuesday in an unannounced diplomatic mission to reassure Ukraine that it has American support as it struggles to defend against increasingly intense Russian attacks.

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.

On his fourth trip to Kyiv since Russia invaded Ukraine in February 2022, Blinken will underscore the Biden administration's commitment to Ukraine's defense and long-term security, U.S. officials said. They noted 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 longer-term support.

It is "trying to really accelerate the tempo" of U.S. weapon shipments to Ukraine, national security adviser Jake Sullivan said.

"What I am going to suggest is that the level of intensity being exhibited right now in terms of moving stuff is at a 10 out of 10," Sullivan told reporters at a White House briefing Monday.

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.

Blinken will "send a strong signal of reassurance" to Ukrainian leaders and civil society figures he will meet during his two-day visit, said the official, who spoke to reporters on condition of anonymity ahead of Blinken's meetings.

In a statement released after Blinken's arrival, the State Department said he would meet with President Volodymyr Zelenskyy and other top Ukrainian officials "to discuss battlefield updates, the impact of new U.S. security and economic assistance, long-term security and other commitments, and ongoing work to bolster Ukraine's economic recovery."

Delays in U.S. assistance, particularly since Israel's war with Hamas began to preoccupy top administration officials, triggered deep concerns 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.

The U.S. official added that Blinken also would 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 Russian President Vladimir 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 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.

Top Biden administration officials and Ukrainian national security officials held a call Monday "about the situation on the front, about the capabilities that they are most in need of, and a real triage effort to say, 'Get us this stuff this fast so that we can be in a position to effectively defend against the Russian onslaught," Sullivan said.

Zelenskyy said over the weekend that "fierce battles" are taking place near the border in eastern and northeastern Ukraine as outgunned and outnumbered Ukrainian soldiers try to push back a significant Russian ground offensive.

The Kremlin's forces are aiming to exploit Ukrainian weaknesses before a big batch of new military aid for Kyiv from the U.S. and European partners arrives on the battlefield in the coming weeks and months, Ukrainian commanders and analysts say. That makes this period a window of opportunity for Moscow and one of the most dangerous for Kyiv in the two-year war, they say.

The new Russian push in the northeastern Kharkiv region and a drive into the eastern Donetsk region come after months when the roughly 1,000-kilometer (620-mile) front line barely budged. In the meantime, both sides have used long-range strikes in what largely became a war of attrition.

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 surge.

Blinken said Sunday that there was "no doubt" the monthslong delay in aid caused problems but that "we are doing everything we can to rush this assistance out there."

"It's a challenging moment," he told CBS' Face the Nation. "We are not going anywhere, and neither are more than some 50 countries that are supporting Ukraine. That will continue, and if Putin thinks he can outlast Ukraine, outlast its supporters, he's wrong.''