Russian forces pressed ahead Friday with an offensive into northeast Ukraine but President Vladimir Putin said there were no current plans to occupy the key city of Kharkiv. 

On a trip to China, Putin said the latest assault was direct retaliation for Ukraine's shelling of Russia's border regions and his country was trying to create a "security zone".

"This is their fault because they have shelled and continue to shell residential neighbourhoods in border areas," Putin told reporters.

He added there was no intention, at this stage, to take Kharkiv, Ukraine's second city, about 30 kilometres (18 miles) from the border. More than one million people still live there.

Russia launched the surprise offensive into Ukraine's northeast on 10 May, sending thousands of troops across the border and unleashing artillery fire on several settlements.

Both countries said Russian troops were still advancing, but Ukraine warned heavy fighting lay ahead.

Russia's defense ministry said its army had "liberated 12 settlements in the Kharkiv region over the last week... and continues to advance deep into enemy defences."

Russian forces took 278 square kilometres (107 square miles) -- their biggest gains in a year-and-a-half -- between May 9 and 15, AFP has calculated using data from the Institute for the Study of War (ISW).

Most has been in the northeast, though its troops have also advanced in southern Ukraine.

Heavy fighting

The Ukrainian governor of the Kharkiv region, Oleg Synegubov, said Russian forces were trying to surround Vovchansk, an almost deserted town which had a pre-war population of around 18,000.

"The enemy has actually started to destroy the town. It is not just dangerous to be there, but impossible," Synegubov told a briefing.

He said Ukrainian troops were resisting, but warned Russia was gaining ground near Lukyantsi, a village around 20 kilometres (12 miles) northeast of Kharkiv city.

Kyiv pulled its troops back from that area this week amid heavy fire and has rushed in reinforcements.

Ukraine army chief Oleksandr Syrsky said Russia was trying to force Ukraine to pull up even more troops from its reserves.

"We realize that there will be heavy fighting ahead and the enemy is preparing for it," he said.

Russia has a manpower and ammunition advantage across the front lines, and military analysts say the fresh offensive could aim to further stretch Ukrainian troops and resources.

Ukraine has evacuated almost 9,000 people from the area since Russia launched the offensive.

The city of Kharkiv has been targeted with intense Russian aerial bombardments, including the weeks ahead of the ground assault.

A rocket strike on the city on Friday killed two people and injured at least 13, Synegubov said in a post on Telegram.

Drone wave

Putin's comments about the need to protect Russian border zones came as Ukrainian drone and shelling attacks on border regions killed at least three people, including a child.

Kyiv launched one of its largest aerial attacks in weeks, firing drones at Russia and the annexed Crimea peninsula overnight.

The Russian military said it intercepted or downed more than 100 Ukrainian drones over the south of the country, Crimea and the Black Sea.

Officials in multiple Russian regions reported damage.

One drone struck a family driving near the border in the Belgorod region, killing a mother and her four-year-old son, the region's governor Vyacheslav Gladkov said.

"The child was in a critical condition. Doctors did everything possible to save him. (But) to much grief, the four-year-old died in hospital," he said.

Shelling on another border village in Russia's Bryansk region on Friday killed one person, the regional governor said.

In the coastal town of Tuapse in the southern Krasnodar region, Ukrainian drones hit an oil refinery for the second time this year, sparking a large fire that was put out, authorities said.

Several fires also erupted after a drone attack on Novorossiysk, a key port city also in the Krasnodar region, local governor Veniamin Kondratyev said.

A source in Ukraine's defense sector confirmed Kyiv had targeted oil facilities in both cities, and also hit an electrical substation in the Russian-controlled port of Sevastopol on the annexed Crimean peninsula.

The city's Russian-installed governor Mikhail Razvozhayev said there had been a "partial blackout" after debris from downed drones damaged a substation.