GENEVA — More than 1 million people driven out of their countries by war, poverty and persecution entered Europe in this record-breaking year, migration experts said Tuesday, a symbolic milestone capping a mass movement of people that has challenged the concept of European unity.

With just days left in 2015, the Geneva-based International Organization for Migration said 1,005,504 people had entered Europe as of Monday, more than four times as many as last year. Almost all came by sea, while 3,692 others drowned trying to make the crossing.

IOM director-general William Lacy Swing urged European governments to make migration safer.

“We know migration is inevitable. It’s necessary and it’s desirable,” he said, adding: “Migration must be legal, safe and secure for all – both for the migrants themselves and the countries that will become their new home.”

The IOM compiles the numbers from government records in Greece, Italy, Bulgaria, Spain, Malta and Cyprus, spokesman Joel Millman said. He noted that the real number of people entering Europe may be even larger, because authorities are struggling to track all arrivals given the sheer volume.

Most people entered Europe via Greece, which took in 820,000 people this year, nearly all of them crossing from Turkey by boat across the Aegean Sea. Another 150,000 came into Italy across the Mediterranean from north Africa, while smaller numbers crossed from Turkey by land into neighboring Greece and Bulgaria. Much smaller numbers arrived by boat to other Mediterranean countries.