KYIV — A massive Russian column was closing in on Kyiv Tuesday, with US officials warning Russia's sheer numbers could be able to overcome the Ukrainian resistance.

The 40-mile-long (64-kilometer) Russian military convoy made up of armored vehicles, tanks, towed artillery and other logistical vehicles has reached the outskirts of Ukraine's capital, according to satellite images from Maxar Technologies. Maxar said it saw plumes of smoke rising from a number of homes and buildings near the roads where the convoy is traveling, although it's unclear what the cause was.

The new images come as US officials told lawmakers in classified briefings Monday that a second wave of Russian troops will likely consolidate the country's positions within Ukraine, and by sheer numbers could be able to overcome the Ukrainian resistance, according to two people familiar with the briefings.

"That part was disheartening," one lawmaker told CNN.

The stark warning came as Ukraine appealed to the international community to come to its aid, negotiations were held between Russian and Ukrainian officials, and Moscow scrambled to prevent financial meltdown as its economy was slammed by sanctions imposed over the invasion.

Already, more than 400 civilians have been killed or injured since Moscow's unprovoked assault on its neighbor began Thursday, according to the United Nations, and Ukraine's leader has accused Russia of committing war crimes by targeting civilians.

But US officials fear the worst is yet to come. US officials who were previously surprised by the fierce Ukrainian resistance that saw regular citizens take up arms now fear that the situation is becoming "a lot more challenging" for the Ukrainians.

US officials told the briefing Monday that Russia would likely lay siege to Kyiv, leading to ugly scenes of urban warfare, one of the people familiar with the matter said.

In Kherson, where the Ukrainian military resisted a Russian onslaught for days, Ukrainian defensive lines appear to have fallen and Russian military vehicles have now been seen driving inside the city.

After a meeting on Capitol Hill on Monday where the Ukrainian ambassador to the US requested more weapons, high-ranking US Republican Senator Jim Risch said Ukraine was struggling.

"It's David versus Goliath," he said.

The Russian onslaught also raises fears for the safety of civilians, who have already been targeted by Russian forces, according to Ukraine.

Ukraine has accused Russia of committing war crimes by targeting civilians, and on Monday, the International Criminal Court said it would open an investigation into Russia's invasion of Ukraine -- a move that was welcomed by Kyiv.

ICC Prosecutor, Karim A.A. Khan said in a statement that, following a preliminary examination into the situation, there is a reasonable basis to "believe that both alleged war crimes and crimes against humanity have been committed in Ukraine."

Russia maintains that it isn't targeting civilian infrastructure in Ukraine, and that there is no evidence of civilian deaths caused by the Russia military. Russia's outgoing President of the UN Security Council Vassily Nebenzia repeated these claims on Monday, stating the "tide of dirty lies replicated in Western mass media unfortunately have become a dangerous mark of our time."

But there is a growing body of evidence to show that civilians are being targeted, and the UN said Monday that 406 civilian casualties in Ukraine have been reported.

Russian forces bombarded a residential area in Ukraine's second-largest city, Kharkiv, with rockets on Monday, killing nine civilians, including three children, and wounding 37 others, the city's Mayor Ihor Terekhov said. CNN has reached out to Russian authorities for comment on the attack.

In a late night address Monday, Ukrainian President Volodymyr Zelensky said the attack on Kharkiv was "clearly a war crime."

"Kharkiv is a peaceful city, there are peaceful residential areas, no military facilities. Dozens of eyewitness accounts prove that this is not a single false volley, but deliberate destruction of people. The Russians knew where they were shooting.

"No one in the world will forgive you for killing peaceful Ukrainian people," he added.

Russia's shelling of Ukraine continued during negotiations held between the two countries Monday, with Zelensky saying the attacks had been "synchronized" with the five-hour talks.

"There can be fair negotiations if one side does not hit the other side with rocket artillery at the time of negotiations," he said in a Facebook message. "I think that with this simple-minded method Russia is trying to pressurize."

Both sides discussed a potential "ceasefire and the end of combat actions on the territory of Ukraine," Ukrainian presidential adviser Mikhaylo Podolyak told reporters. Without going into detail, he said both sides would return to their capitals for consultations over whether to implement a number of "decisions."

As bitter fighting takes place across the country, many Ukrainians are fleeing to safety.

There are already 520,000 refugees from Ukraine in neighboring counties, with the figure rising "exponentially, hour after hour," Filippo Grandi, the UN High Commissioner for Refugees (UNHCR), told the UN Security Council on Monday.

Many of them are women who have had to make tough decisions to leave behind their fathers and husbands -- men ages 18 to 60 are banned from leaving Ukraine.

Foreigners are also desperately trying to leave the country. In the Ukrainian village of Shehyni on the border with Poland, foreign nationals are having to wait in the cold for hours to get out, with some even lighting fires to keep warm.

Some foreign students attempting to leave told CNN they faced racist treatment by Ukrainian security forces and border officials.

"Ukrainians have been prioritized over Africans -- men and women -- at every point," Rachel Onyegbule, a Nigerian first-year medical student in Lviv told CNN in a telephone call Sunday as she waited in line at the border to cross into Poland.

Some Ukrainians have opted to stay and join the resistance. Volunteers are pouring into the capital, where there's a feeling of defiance among many. Some are gathering bottles to make Molotov cocktails.

Newlyweds Yaryna Arieva and Svyatoslav Fursin have spent their honeymoon fighting to defend their country.

"No one here is saying that we will lose, or is crying," Arieva, who is from Kyiv, told CNN. "Everyone here believes we will win. It's all just a question of time. So, I am very happy to see this great amount of people, really being ready to fight. Being ready to kill for their land. Having no doubt about our win in this war."

Nevertheless, the picture for those in Ukraine is "grim" -- and could get worse, UN Office for the Coordination of Humanitarian Affairs (OCHA)'s Martin Griffiths said Monday.

"Aerial attacks and fighting in urban areas are damaging critical civilian facilities and disrupting essential services such as health, electricity, water and sanitation, which effectively leaves civilians without the basics for day-to-day life," said Griffiths, OCHA's under-secretary-general for humanitarian affairs and emergency relief coordinator.

He said the real number of civilian casualties "could be considerably higher, as many reported casualties have yet to be confirmed."

As Russia's brutal attack on Ukraine continues, Kyiv is desperately requesting further support from international powers.

Oksana Markarova, the Ukrainian ambassador to the US, told a bipartisan group of lawmakers at Capitol Hill Monday that her country needs more weapons and other assistance in its existential fight against Russia.

"We are not asking anyone to fight for us, we are defending our country ourselves. But we need all the support that all civilized world can give us to actually continue effectively fighting, and also sanctions," she said after the meeting.

In recent days, US President Joe Biden instructed Secretary of State Antony Blinken to release up to $350 million to immediately support to Ukraine's defense -- but officials have also acknowledged privately that it will be more difficult to get new aid to Ukraine than it was previously when it could be flown directly to Kyiv.

Australia will send missiles as part of a $50 million package of lethal and non-lethal aid to help Ukraine repel Russian forces, the country's Prime Minister Scott Morrison told a news conference Tuesday. — CNN