The United States will back Ukraine until its security is "guaranteed", US Secretary of State Antony Blinken vowed on a trip to Kyiv on Tuesday, after Russian forces claimed further advances in the northeastern Kharkiv region. 

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

"We're with you today. And we will stay by your side, until Ukraine's security, sovereignty, and ability to choose its own path is guaranteed," Blinken said in a speech. 

"The assistance is now on its way. Some of it has already arrived. More will be arriving," he had told Ukrainian President Volodymyr Zelensky during a meeting earlier. 

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

Zelensky thanked Washington for the aid, which he said was "crucial" — but stressed it was "important to get it as quickly as possible". 

"We need a noticeable acceleration of supplies. Now too much time passes between the announcement of packages and the actual appearance of weapons on the frontline," he said in an evening address. 

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 along the border since Friday. 

Blinken was on his fourth visit to Kyiv since Russia invaded Ukraine in February 2022. 

After meeting Zelensky, he met Ukrainian Foreign Minister Dmytro Kuleba in Veterano, a pizzeria set up by a military veteran in the city centre. 

Driving his point home at a visit to an underground Kyiv bar, the Barman Dictat, later on Tuesday, Blinken picked up a guitar and sang the Neil Young classic "Rockin' in the Free World", accompanied by Ukrainian musicians.

'Minimal human losses'

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

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

Officials in the region's main municipality, also called Kharkiv -- Ukraine's second-biggest city — said Russian strikes hit the centre, injuring 20 people.

One civilian, a 47-year-old man, was also reported killed in the city of Nikopol in southern Ukraine.

Russia's defence ministry meanwhile said its forces had captured another village in the Kharkiv region.

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

The advance is the latest in a string of tactical successes for Russia on the battlefield this year after initial setbacks in a conflict that Moscow hoped would be wrapped up in days.

Russia's incoming defence minister, economist Andrei Belousov, said the priority was to secure victory on the battlefield while minimising human losses.

"The key task, of course, remains achieving victory and ensuring the military-political goals of the special military operation, set by the president, are achieved," Belousov told a session of the Russian parliament on Tuesday.

"In this respect, I want to especially emphasise 'with minimal human losses'."

'Firing on everything'

The Ukrainian army has acknowledged Russian successes in Kharkiv but Zelensky has stressed that reinforcements have been sent there and Ukrainian "counterattacks are ongoing".

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", added Volodymyr Usov, head of the Kharkiv district military administration.

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

In an interview this week with AFP, he said Russia had sent more than 30,000 troops over the border, but insisted there was no threat of an assault on Kharkiv city.

Usov, the Kharkiv district head, estimated there were still around 300 residents left in Lyptsi, a border village under Russian bombardment.

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

State power operator Ukrenergo said it was forced to introduce emergency blackouts in all regions of the country on Tuesday after weeks of Russian strikes on power plants left the country unable to meet demand during a cold snap.