More Than 1 Million Ukrainians Have Fled Russia's Invasion, U.N. Says, with Hundreds of Civilian Deaths

More than a million refugees have fled Ukraine since Russia began its attack seven days ago, according to the United Nations.

"In just seven days we have witnessed the exodus of one million refugees from Ukraine to neighbouring countries," the U.N. High Commissioner for Refugees (UNHCR), Filippo Grandi, said in a tweet on Wednesday. "For many millions more, inside Ukraine, it's time for guns to fall silent, so that life-saving humanitarian assistance can be provided."

An agency spokesperson said the situation "looks set to become Europe's largest refugee crisis this century" while commending Ukraine's neighbors for keeping their borders open to Ukrainians fleeing Russian attacks.

"We have seen tremendous solidarity and hospitality from the countries receiving refugees, including from the authorities and local communities," Shabia Mantoo said in a statement.

The UNHCR said an estimated 4 million people may flee Ukraine, whose total population is 44 million, in the coming weeks and months.

Most are women, children and elderly. Men ages 18 to 60 are reportedly prohibited from leaving after President Volodymyr Zelenskyy asked Ukrainians to join the effort to defend their country.

"You don't know where to go, where to run, who you have to call," Liliya Marynchak, a 45-year-old teacher in Ivano-Frankivsk, Ukraine, told PEOPLE recently of the moment her city was bombed.

"This is just panic," she said.

Over half of the million who have fled Ukraine arrived in neighboring Poland to the west, according to data provided by the U.N. There are more than 130,000 in Hungary and almost 100,000 in the Republic of Moldova.

"We will do everything to provide safe shelter in Poland for everyone who needs it," the country's interior minister, Mariusz Kaminski, said at the start of the conflict.

The number of civilians killed in the attacks is difficult to determine accurately, but initial data shows a stark toll.

On Wednesday the U.N.'s High Commission for Human Rights published its civilian casualty report claiming 227 were killed, including children, and 525 were injured but warned "real figures are considerably higher."

The commission said most of the casualties were caused by "explosive weapons with a wide impact area, including shelling from heavy artillery and multi-launch rockets systems and air strikes."

Ukraine's State Emergency Service said Wednesday more than 2,000 Ukrainians have died "not counting our defenders," CNN reports.

"Children, women and our defense forces are losing their lives every hour," the statement added.

Details of the invasion and the fighting change by the day, but this is the first major land conflict in Europe in decades.

The attack, ordered by Russian President Vladimir Putin, has drawn widespread condemnation around the world and increasingly severe economic sanctions against Russia.

In addition to the economic sanctions imposed by a global coalition that opposes Putin's aggression, various countries have also pledged aid or military support to Ukraine.

Putin insists the country has historic ties to Russia and he is acting in the interest of so-called "peacekeeping."

"The prayers of the entire world are with the people of Ukraine," President Joe Biden said as the invasion began in force in February.