Blinken's trip comes just weeks after the US Congress finally approved a $61 billion financial aid package for Ukraine after months of political wrangling, unlocking much-needed arms for the country's stretched troops.

"The assistance is now on its way. Some of it has already arrived, more will be arriving," Blinken told Ukrainian President Volodymyr Zelensky.

"That's going to make a real difference against the ongoing Russian aggression on the battlefield," he said.

Zelensky thanked Washington for the aid, saying it was "crucial" and added that it was "important to get it as quickly as possible".

Zelensky said air defence was the "biggest problem" for Ukraine and requested two Patriot batteries for the Kharkiv region, where Russian forces have been advancing and pounding villages all along the border since Friday.

Russia's defence ministry on Tuesday said in a statement its forces had captured another village in the region.

"Units of the North group of troops liberated the village of Bugruvatka in the Kharkiv region and advanced deep into the enemy defence," the ministry said.

At a checkpoint outside the city of Kharkiv, a Ukrainian official said Russian forces had entered Ukraine through "villages on the very border that were complicated for us to defend".

Russian forces "are on high ground and are shelling us from there," said the official, Volodymyr Usov, head of the Kharkiv district military administration.

- 'Intense enemy fire' -

Russia's surprise ground offensive in the Kharkiv region has forced thousands to evacuate and pushed Kyiv to mobilise reinforcements.

Ukraine says several civilians have been killed by Russian fire in the region, including on Tuesday two aged 80 and 83.

The Ukrainian army has acknowledged Russia is "achieving tactical success" but Zelensky has stressed Kyiv has sent reinforcements to Kharkiv and that "our counterattacks are ongoing".

"We are getting more and more results, destroying the occupier's infantry and machinery," the president said in a Monday evening address.

"Our task is crystal clear: to thwart Russia's attempt to expand the war".

Zelensky said Kyiv had also noted strikes and "hostile activity" targeted against the Sumy and Chernigiv regions in northern Ukraine.

In the east of the country, where several areas had come under "intense enemy fire", Ukrainian forces had changed their positions "to save the lives of our defenders" and were planning to regroup units, the army said late on Monday.

Ukraine's security council chief Oleksandr Lytvynenko said Moscow had massively upped its troop deployment for the new offensive in the Kharkiv region.

"There are a lot of Russians, quite a lot. There were about 50,000 on the border. Now there are many more than 30,000 coming," he told AFP on Monday.

However, he said "we don't see any threat of assault on the city of Kharkiv", around 30 kilometres (20 miles) from the combat zone.

- Bombs falling 'like rain' -

Kateryna Stepanova, 74, who fled Lyptsi with her son, said several bombs had hit her street.

"We weren't going to leave... but now this. Thankfully, we're alive," she said, sitting in a minibus at a gathering point for evacuees.

"They are shelling the villages, firing everything they can," Sergiy Kryvetchenko, deputy head of the Ukrainian military administration in Lyptsi, told AFP.

Guided aerial bombs are falling "like rain" said one serviceman, who was resting after fending off Russian assaults in Lyptsi.

An overnight strike on Kharkiv wounded two people, regional governor Oleg Synegubov said on Tuesday on Telegram.

In Russia, the defence ministry said air defences had intercepted 25 Ukrainian rockets in the Belgorod border region.

Elsewhere, in the southern Volgograd region, freight train carriages were derailed in Kotluban station due to the "intervention of unauthorised persons", Russian news agencies said on Tuesday.

No details were given about the incident or who caused it.

Kyiv did not claim responsibility but a Ukrainian advisor to the mayor of Mariupol, which was seized by Russia in spring 2022, called it "good news".

According to him, the station had a branch line "leading to the arsenal of the main missile and artillery department of the (Russian) ministry of defence".

Kotluban is about 300 kilometres from the border with Ukraine, and the Volgograd region is sometimes the target of drone attacks launched by the Ukrainian army.