GENEVA An estimated 140,000 refugees and migrants reached Europe by sea in November, a big drop from October, although the numbers crossing remain very high and could reach 1 million for the year, the U.N. refugee agency said on Tuesday.

After a brief lull due to rough seas, the numbers have picked up again, with approximately 5,201 landing on the Greek islands on Monday, it said.

"The monthly figure for November is the first one this year that actually shows a decrease compared to the previous one," William Spindler of the U.N. High Commissioner for Refugees (UNHCR) told a news briefing. Some 220,000 were recorded in October.

"The reasons for the slowdown in the number of arrivals have to do with fluctuating climate conditions in the Aegean but also a crackdown on smuggling by Turkish authorities."

So far, 886,662 people seeking safety have reached European shores this year, about four times the total in 2014, he said. Half of those arriving are Syrians fleeing war. The vast majority reaching Europe arrive by sea.

"It is not unrealistic to say we could reach 1 million by the end of the year," Spindler said.

Women and children now form just over half of the refugees and migrants on the move, or 52 percent, the U.N. Children's Fund (UNICEF) and the International Organization for Migration said.

On Saturday, police in Macedonia fired teargas and stun grenades at desperate migrants stranded for days at the Greek border and who are demanding passage to western Europe, as soldiers began erecting a metal fence to keep them out. Only Syrians, Iraqis and Afghans are allowed through.

Hundreds of children are among those trapped, including nationals of Lebanon, Pakistan, Iran and Bangladesh, UNICEF spokesman Christopher Tidey, on a tour of the region, said by telephone. Children are highly vulnerable to respiratory infections as winter bites, he said.

"What I'm seeing basically throughout the course of the journey is that children on the move are spending far too much time in the open without enough rest," he said.

Referring to the people stranded on the Greek border with Macedonia, Tidey said: "They are living in extremely poor conditions with inadequate shelter, people are sleeping sometimes 30 to 40 inside of a tent to keep warm."

"A lot of families that we spoke with had been there seven to 10 days and that includes families with children."

(Reporting by Stephanie Nebehay; Editing by Alison Williams)