Huge Russian convoy approaches Kyiv as fears of all-out assault on the city grow

Russia appears to be advancing in its invasion of Ukraine with satellite imagery indicating that a huge convoy — some 40 miles (65 kilometers) long — of Russian military vehicles is heading toward Ukraine's capital of Kyiv.

The following satellite image was taken by U.S firm Maxar Technologies on Monday and appears to show a convoy of Russian armored tanks and trucks that stretches from Pybirsk, further north of Kyiv, to the Antonov airport (also known as the Hostomel airport — the site of fighting last week between Russian and Ukrainian forces) on the northeast outskirts of the Ukrainian capital.

In some parts of the convoy the vehicles are traveling three or four abreast on the road, although in other images the vehicles are seen further apart, in single file. The distance along the road is approximately 40 miles.

It should be noted that the situation in Ukraine is fast moving and may now may be different to what is seen in these images, which show the convoy on Sunday and Monday.

The following two images were taken by Maxar Technologies on Sunday. At this point, the convoy does not appear to cover as large an area and cloud cover precludes a complete view of the area.

U.K. Deputy Prime Minister Dominic Raab said Tuesday that Britain would do "everything is can to delay the fall of Kyiv." The BBC reported Tuesday morning that the convoy is 17 miles from Kyiv, although it's hard to gauge how quickly the military column could reach the city.

Russia began its invasion of Ukraine last Thursday and its forces have attacked various cities and areas in the north, east and south of the country. However, analysts have suggested that Russia has made slower progress in its advance into the country and has met more resistance than it expected.

The Pentagon has noted the Kremlin still wants its troops to capture Kyiv, despite the stiff Ukrainian resistance.

"We have every indication that they still want to take Kyiv, that they are advancing on the ground and trying to get closer," a senior Defense official told CNBC on Monday, speaking on condition of anonymity in order to share new details from the Pentagon's assessment.

If Russia is about to launch a much harder assault on the capital Kyiv, how Ukraine's armed forces and civilian resistance will cope is much more uncertain.

Jack Watling, a research fellow for Land Warfare at the Royal United Services Institute in London, painted a bleak outlook for Kyiv and its remaining residents and resistance fighters, as he described what tactics the approaching Russian forces could use against the city.

"You'll see the city being surrounded, you'll see the key utilities and critical national infrastructure being seized so they [the Russians] control the supply of water and electricity and then intense shelling into the areas where resistance is concentrated followed by combined arms assaults with armor, engineers and infantry," he told the BBC's "Today" program on Tuesday.

Watling said the West had to acknowledge that it had tried to help Ukraine with the supply of weapons and so forth, "far too late."

"The warnings about this were in place months ago and anything we do now, including the arms that have been sent over the last few days, which are essentially building up in Poland so it would take a significant amount of time to get them — without them being hit by Russian aircraft — all the way across Ukraine and into the hands of the defenders and that's a policy failure on our part."

Elsewhere on Tuesday, Ukraine's second-largest city, Kharkiv, has come under heavy shelling with a massive explosion hitting the Regional State Administration building, according to videos shared on social media and by Ukrainian officials.

Ukraine's State Emergency Services said the explosion was caused by a Russian airstrike on the city and an Interior Ministry advisor said in a post on social media that at least 10 people were killed and 35 wounded Tuesday in rocket strikes by Russian forces. NBC News was unable to independently verify the claims.

Ukrainian President Volodymyr Zelenskyy has issued a video statement calling the shelling of Kharkiv a "military crime."

There are also concerns that Belarus is preparing to assist Russian troops in their invasion, with an official Ukrainian Twitter account saying Tuesday that Belarusian troops had entered the Chernihiv region in northern Ukraine, and near the Belarusian border. It said the information was confirmed by a spokesman for the North Territorial Defense Forces. It has not been verified by NBC News.

Other images from Maxar taken Monday suggested that additional military activity was taking place in southern Belarus, which borders Ukraine and is an ally of Russia, with ground forces and ground attack helicopter units seen in the images.

Again, official sources have not confirmed whether these units — or Belarus more broadly — is preparing to join Russian forces in an assault on Ukraine. Belarus' president, Alexander Lukashenko, had said earlier Tuesday that his country has no plans to join the incursion, according to state news agency Belta.

Peace talks were held between Russian and Ukrainian delegates on Monday in neighboring Belarus, and although no breakthrough was reached, there are hopes that talks will continue in the coming days.

For its part, Ukraine has vowed that it will not surrender to Russia and has demanded an immediate cease-fire and that Russian forces leave its territory.

"Ukraine is ready to continue seeking a diplomatic solution, but Ukraine is not ready to surrender or capitulate," Foreign Minister Dmytro Kuleba told CNBC's Hadley Gamble on Monday.

Zelenskyy had already struck a pessimistic tone over the outcome of talks, and Kuleba told CNBC he did not know whether the discussions would be successful.

"I'm a diplomat, I have to believe in the success of talks, but at the same time my main goal as a diplomat now is to impose more sanctions on Russia, to bring more weapons to Ukraine and to isolate Russia as much as we can in the international arena so I'm focused on this part of diplomacy," Kuleba said.

"We stand not only for ourselves but for the world order as we all know it."

The foreign minister said that "every 24 hours are crucial, because Russia deploys more and more military might on Ukraine, hundreds of tanks moving into the country, they dominate the skies with their bombers, rockets, missiles are being thrown on our peaceful cities, civilians being killed, but we continue fighting."

He said the war had turned into a "real people's war against Russian aggression."

Russia has already conducted attacks both on the ground and by air, with major cities reporting shelling and damage to buildings. Both sides have said that some of their soldiers have been killed, and Ukraine has reported that several hundred of its citizens have died during Russian attacks, including a number of children.

The invasion has led to a huge number of Ukrainians attempting to flee the country for safety in Eastern Europe, although many have also stayed to defend their homes and nation. A spokesperson from UNHCR, the U.N. refugee agency, confirmed to NBC News on Tuesday that more than 660,000 people have fled from Ukraine into neighboring countries in the past six days.

 — CNBC's Amanda Macias and Ted Kemp contributed reporting to this story.