At least one million people have fled Ukraine in the week since Russia’s invasion, the United Nations said, with one official warning that “at this rate” the exodus could become “the biggest refugee crisis this century”.

Thursday’s tally from the UN High Commissioner for Refugees (UNHCR) amounts to more than 2 percent of Ukraine’s population, which the World Bank counted at 44 million at the end of 2020, on the move across borders in just seven days, Al Jazeera reports.

The agency cautioned that the outflows were far from finished: It has predicted that as many as 4 million people could eventually leave Ukraine, and even that projection could be revised upward.

Joung-ah Ghedini-Williams, a spokeswoman for the UNHCR, told the Associated Press news agency in an email that “our data indicates we passed the 1 million mark” as of midnight in central Europe, based on counts collected by national authorities.

On Twitter, UN High Commissioner Filippo Grandi wrote, “In just seven days we have witnessed the exodus of one million refugees from Ukraine to neighboring countries.”

He added, “For many millions more, inside Ukraine, it’s time for guns to fall silent, so that life-saving humanitarian assistance can be provided.”

In comparison, Syria, whose civil war erupted in 2011 and remains the country with the largest refugee outflows, had nearly 5.7 million people fleeing, according to UNHCR’s figures. But even at the fasted rate of flight in early 2013, it took at least three months for 1 million refugees to leave Syria.