Hungary and Croatia traded threats on Saturday as thousands of exhausted migrants poured over their borders, deepening the disarray in Europe over how to handle the tide of humanity.

More than 20,000 migrants, many of them refugees from the Syrian war, have trekked into Croatia since Tuesday, when Hungary used a metal fence, tear gas and water cannon on its southern border with Serbia to bar their route into the European Union.

EU leaders, deeply divided, are due to meet on Wednesday in a fresh attempt to agree on how and where to distribute 160,000 refugees among their countries, but the noises from some of the newer members of the bloc were far from friendly.

Hungary, where the right-wing government of Viktor Orban has vowed to defend "Christian Europe" against the mainly Muslim migrants, accused Croatia of "violating Hungary's sovereignty" by sending buses and trains packed with migrants over their joint border. It warned it might block Zagreb's accession to Europe's Schengen zone of passport-free travel.

"Croatia's government has continuously lied in the face of Hungarians, Croatians, of the EU and its citizens," Foreign Minister Peter Szijjarto told a news conference. "What kind of European solidarity is this?"

Croatian Prime Minister Zoran Milanovic said that, unlike Hungary, he would not use "brute force" to keep people out, nor would his government make them stay against their will. The buses and trains would keep running to Hungary, he said.

"We forced them (to accept the migrants), by sending people up there. And we'll keep doing it," he told reporters.

DEATHS AT SEA

Croatia, a country of 4.4 million people forged as an independent state in a 1991-95 war, has suddenly found itself in the way of the largest migration of people westwards since World War Two. On Friday, Milanovic said the country could not cope, and would simply wave the migrants on.

Almost 500,000 people have crossed the Mediterranean to Europe so far this year, increasingly across the water from Turkey to Greece and then up through the impoverished Balkans to the former Yugoslavia, of which Croatia and Slovenia are members of the EU.

Many are rushing to beat rougher seas; on Saturday, a girl believed to be five years old died and 13 other migrants were feared drowned when their boat sank off the island of Lesbos.

A second, exhausted group of around 40 people reached the inundated island in a tiny dinghy following a traumatic journey from Turkey, having paddled through the night with their hands across 10 km (six miles) of sea when their engine failed.

"When we were on the sea ... I didn't have any hope ... I said: I am dead right now, nobody can help me," 18-year-old Mohammed Reza said after being helped ashore by foreign volunteers.

Nearly 4,700 migrants were rescued off the coast of Libya on Saturday as they tried to reach Europe but one woman was found dead on board a boat, Italy's coastguard said.

July and August alone brought 150,000 migrants to Greek shores, about as many as the EU says it is planning to accommodate if it can overcome the opposition of many newer members of the bloc in ex-Communist eastern Europe. Hungary and Slovenia are among those opposed to the quotas Germany and others in northern and western Europe are calling for.

The vast majority of refugees want to reach Germany, which has said it expects to receive 800,000 asylum seekers this year.

They kept coming on Saturday, crammed onto bus and train having crossed into Croatia from Serbia and driven north and west towards Hungary and Slovenia. Many spent the night under open skies, and the day searching for shade from a scorching late summer sun.

Hungary said some 8,000 had arrived from Croatia on Friday, with more on their way. Most were sent to reception centres near Hungary's border with Austria, which in turn said about 7,500 had entered since midnight, with more to follow.

SYRIAN WAR

Hungarian soldiers are racing to build a fence along the Croatian frontier like the one erected the length of its border with Serbia. The government said on Saturday it had called up some army reservists, mostly to staff garrisons left empty by soldiers deployed to the border.

"If Croatia puts up its hands and says, no, I don't want to defend the borders, then Hungary can only say that it isn't ready to join Schengen when the moment comes to decide," Antal Rogan, an adviser to Orban, told InfoRadio news station.

Crowds were building too on Croatia's border with Slovenia, which like Hungary is a member of the Schengen zone. Police were rationing entry to small groups, mainly families, and Slovenian Prime Minister Miro Cerar suggested he may have to discuss with neighbouring states the creation of a "corridor" to allow their passage through the tiny country of 2 million people. Some 2,000 had entered by Saturday.

"I feel frustrated, we're so tired," said Siha, 35, from Aleppo, Syria's commercial hub, parts of which have been reduced to rubble by the four-year-old war. She held close her two young children on a bridge in no-man's-land at the Harmica border crossing into Slovenia.

"We left Turkey 10 days ago. The trip was very dangerous for the kids. I decided to leave Syria because I want my kids to have a comfortable life, to study," she said.

An estimated three million Syrians have fled the war and many more are displaced inside their country, feeding the tide to western Europe. U.S. Secretary of State John Kerry said there was an urgent need to renew efforts to find a political solution to the war and the worsening refugee crisis.

"We need to get to the negotiation. That is what we're looking for and we hope Russia and Iran, and any other countries with influence, will help to bring about that, because that's what is preventing this crisis from ending," Kerry said after talks in Londonwith British Foreign Secretary Philip Hammond.

The United States has taken in 1,500 refugees since the start of the Syrian war in 2011, and President Barack Obama last week committed to accepting 10,000 more over the coming year.

Anne Richard, assistant secretary of state for population, refugees and migration, said on Saturday she would like to see a steep increase in numbers in 2017.

The government of Iceland, with a population of only 330,000, said it planned to spend 2 billion Icelandic crowns ($15.9 million) this year and next on aid to people fleeing the Syrian crisis. Iceland would take about 100 refugees this year.