KHARKIV - Satellite images collected and analysed by Maxar Technologies show an eight-mile-long military convoy moving south through the eastern Ukraine town of Velkyi Burluk on Sunday. The town sits to the east of Kharkiv, close to Ukraine’s border with Russia.

The images show “armored vehicles, trucks with towed artillery and support equipment” making up the convoy, Maxar said. Ukrainian officials say major fighting is underway in the east of the country, with heavy shelling reported throughout the Donbas region, ahead of what they are warning may be a major Russian offensive. Ukraine’s defense intelligence chief on Sunday told CNN that Russian troops are regrouping across the border and plan to advance toward Kharkiv. Officials have urged the evacuation of civilians from the region, as Russian forces shift focus to southern and eastern Ukraine.

Earlier in Russia’s invasion, a 40-mile-long (64-kilometer) Russian military convoy, composed of tanks, armored vehicles and towed artillery, sparked dread among defending Ukrainians as it lumbered towards their capital Kyiv in northern Ukraine. But the convoy stalled as it faced tough Ukrainian resistance and challenges in accessing supplies, before dispersing without ever reaching the city.

More than 4.5 million Ukrainians flee war: UN

More than 4.5 million Ukrainian refugees have fled their country since the Russian invasion on February 24, according to the United Nations refugee agency.

Images show 8-mile long Russian convoy in eastern Ukraine

The UNHCR said there were 4,503,954 Ukrainian refugees on Sunday. That was 62,291 more than the previous day.

Europe has not seen such a flood of refugees since World War II.

Ninety percent of those who have fled Ukraine are women and children, as the Ukrainian authorities do not allow men of military age to leave.

According to the UN’s International Organization for Migration (IOM), around 210,000 non-Ukrainians have also fled the country, sometimes encountering difficulties returning to their home countries.

A further 7.1 million people have been displaced within the country, according to figures published by the IOM on April 5. That means more than a quarter of the population have been forced to flee their homes, seeking refuge either abroad or elsewhere in Ukraine.

Before the conflict, Ukraine was home to more than 37 million people in territory controlled by Kyiv -- which does not include Crimea, annexed by Russia in 2014, or areas in the east under the control of pro-Russian separatists.

Poland hosts by far the largest number of refugees from Ukraine.

Since the start of the war, 2,593,902 had crossed into Poland as of Saturday, the UNHCR said. For its part, the Polish border police said on Sunday it had recorded 2,630,000 arrivals.

Many refugees travel on to other European countries.

Of those who remain in Poland, 700,000 have already been granted a national identification number, the UNHCR said.  The number is widely used in dealings with Polish public institutions and health services, to obtain a telephone number and access to certain banking services.

Polish border police estimate that more than 500,000 people have returned to Ukraine since the conflict began.