GENEVA — The number of Syrians who have fled into neighboring countries to escape the civil war has reached more than four million, the United Nations said Thursday, and with the fighting dragging into its fifth year the number is still rising.

More than 24,000 people crossed into Turkey to escape fighting in northern Syria in June, pushing the number now sheltering in neighboring countries past four million, increasing the Syrian refugee population by one million in just 10 months, the United Nations refugee agency reported.

International aid agencies say the fighting has driven at least 7.6 million people who remain in the country from their homes.

“This is the biggest refugee population from a single conflict in a generation,” Antonio Guterres, the United Nations high commissioner for refugees, said in a statement. Mr. Guterres, once again, warned that international aid was not keeping pace with the scale of the crisis, and that many refugees were “sinking deeper into poverty.”