The trip by Secretary of State Antony Blinken and Defense Secretary Lloyd Austin -- which the United States confirmed only after the two had left Ukrainian territory -- came as the invasion enters its third month, with thousands dead and millions displaced.
Presidential adviser Oleksiy Arestovych said Ukrainian leader Volodymyr Zelensky met the US officials Sunday, as the port city of Mariupol's defenses were "on the brink of collapse" and Kyiv was in dire need of offensive weapons.
Washington ordered the withdrawal of its diplomats in the weeks prior to Russia's February invasion of Ukraine, but has been a leading donor of financial aid and weaponry to the country, and a key sponsor of sanctions targeting Moscow.
"Since the start of hostilities, we've had a team across the border in Poland who's been handling this work for us," the official told reporters waiting for Blinken and Austin on the Polish side of the border.
"Starting this week, members of that team will be able to do day trips instead into Ukraine," he said.
"Ultimately, (they will) resume presence in Kyiv."
The visiting envoys also pledged another $700 million in military aid to Ukraine, including some $300 million to allow the country to purchase necessary weapons.
The rest of the money will go to Ukraine's regional allies who need to resupply after sending weapons to their neighbour.
The United States has sent some $4 billion in military aid since Biden's term began last year, and already announced Thursday a new $800 million aid package to bolster Ukraine in their fight against Russian troops in the country's east.
But it has been reluctant to intervene militarily, for fear of entering a conflict against nuclear-armed Russia, and officials Monday poured water over claims the visit represented a major escalation of US involvement in Ukraine.
"This visit does not portend actual involvement by US forces," a senior defence official said.
"The president has been very clear there will be no US troops fighting in Ukraine and that includes the skies over Ukraine."
The highly sensitive trip by two of President Joe Biden's top cabinet members coincided with Easter celebrations in the largely Orthodox country.
As Ukrainians marked a sombre Easter, with many braving bombardment for blessings, Russian forces showed no sign of easing attacks.
Five civilians were killed and another five wounded in Donetsk on Sunday, the besieged eastern region's Governor Pavlo Kyrylenko said. Authorities also reported a death in northeastern Kharkiv.
The day before, a missile strike on the southern city of Odessa left eight dead and at least 18 wounded, according to Zelensky, who said five missiles hit the city.
Russia's defence ministry said it had targeted a major depot stocking foreign weapons near Odessa.
Zelensky accused Russia of being a terrorist state, one that has devastated the port city of Mariupol with weeks of unrelenting bombardment.
And with thousands of its fighters and civilians in Mariupol facing increasingly dire conditions, Kyiv said Sunday it had invited Moscow to talks near the sprawling Azovstal steel plant, where Ukrainian soldiers are still holding out.
"We invited Russians to hold a special round of talks on the spot, right next to the walls of Azovstal," the last Ukrainian stronghold in the strategic port, said Arestovych.
There was no immediate response from Russia. Its president, Vladimir Putin, had ordered his forces not to assault the plant, but the Ukrainians say the attacks continue unabated.
Mariupol, which the Kremlin claims to have "liberated", is pivotal to Russia's war plans to forge a land bridge to Russian-occupied Crimea -- and possibly beyond, as far as Moldova.
On Sunday, the United Nations' Ukraine crisis coordinator Amin Awad called for an "immediate stop" to fighting in the city to allow trapped civilians to leave.
"The lives of tens of thousands, including women, children and older people, are at stake in Mariupol," Awad said in a statement after the latest attempt to evacuate civilians from Mariupol failed.
"We need a pause in fighting right now to save lives."
In a message posted on social media Sunday, Sviatoslav Palamar -- deputy commander of the far-right Azov Regiment, which is sheltering in a warren of tunnels under the steel plant -- said Russian forces continued to rain down fire on Azovstal.
"The enemy continues air strikes, artillery from the sea... enemy tanks continue to strike and infantry is trying to storm," said Palamar.
Ukrainian commander Sergey Volyna described the situation in the complex as "very difficult" and reiterated calls for the international community to help those remaining escape.
"We will not have time to wait for a military solution to the situation, the situation is very critical. Very heated. I don't know how much time we have," he said in an interview.