Geneva (AFP) - The number of migrants arriving in crisis-hit Greece is accelerating dramatically, with nearly 21,000 landing on the overstretched Greek islands last week alone, the United Nations said Tuesday.

Since the beginning of the year, more than 160,000 migrants have made their way to Greece -- nearly four times the 43,500 who arrived in the country during all of 2014, the UN refugee agency said.

"The pace of arrivals has been steadily increasing in recent weeks," UNHCR spokesman William Spindler told reporters in Geneva.

Last week, 20,843 migrants -- virtually all of them fleeing war and persecution in Syria, Afghanistan and Iraq -- washed up on the shores of Greek islands, making up nearly half of last year's total.

The wave of migrants is not just impacting Greece, with nearly a third of arrivals in the European Union asking for asylum in Germany. The economic powerhouse is gearing up for nearly three-quarters of a million applicants this year.

"For months, UNHCR has been warning of a mounting refugee crisis on the Greek islands," Spindler said, insisting the "reception infrastructure, services and registration procedures both on the islands and on the mainland need to be strengthened urgently."

Until recently, most migrants making the perilous journey across the Mediterranean to Europe travelled to Italy, but dangers and logistical difficulties have in recent months shifted the flood increasingly towards Greece.

- Situation 'very complicated' -

Shops on the Turkish coast, which is a jumping off point for Greece, have been doing booming business in lifejackets for the migrants making the crossing. One shop owner said she was going through 100-150 of the devices a week.

It is just one of the costs for the migrants, with some paying around $1,200 (1,000 euros) per person for the journey across the Aegean in an inflatable dinghy.

When the migrants arrive on the Greek islands there is little if anything for them and most have been forced to sleep outdoors in squalid conditions, and tensions have been running high

Story continues

The Greek island of Kos, which last week witnessed chaotic scenes of overwhelmed police beating with truncheons some of the thousands of people who had gathered there, has come to symbolise Europe's shambolic response to the refugee crisis.

Greece has taken some steps to address the problem, including sending a large ferry to Kos to serve as a registration and housing facility for refugees.

"The situation is still very complicated," UNHCR emergency coordinator Roberto Mignone told AFP on the island.

He said the authorities' ability to process the refugees remained low, with some 1,750 people currently on the ferry and another 2,500 still on the island.

Spindler insisted the Greek authorities needed to do more to organise the response.

"The government of Greece has the responsibility of what happens on its territory... they need to show much more leadership," he said.

The debt-ravaged country has said the huge influx is too much for it to handle alone and has pleaded for more EU help.

Spindler agreed that Europe needed to do more, adding that "the vast majority" of the migrants aimed to travel on to northern Europe.

- Share the burden -

Germany, as Europe's top economy, has become the refugees' top destination, with one in three who arrived in the EU last year seeking asylum there.

According to a report in the Handelsblatt newspaper, Berlin is bracing for as many as 750,000 refugees to seek asylum in the country this year.

UN refugee chief Antonio Guterres on Tuesday called for more solidarity among European countries in taking in asylum seekers.

The responsibility must be "shared on many shoulders", he told the Die Welt newspaper, insisting it was "unsustainable" for Germany, along with Sweden, to take in the majority of refugees.

There is no sign the flood of migrants into Europe will subside.

Some 250,000 migrants and refugees have already crossed the Mediterranean this year to Italy and Greece, and the International Organization for Migration said Tuesday it expected that number to pass 300,000 by the end of the year.

The ones who make it to shore are meanwhile the lucky ones, with 2,440 people having died trying so far this year, according to UNHCR.

That number includes the 49 migrants asphyxiated in the hold of a ship carrying 362 people that sank at the weekend.

Italy on Tuesday announced the arrest of eight suspected people smugglers accused of condemning the victims to their deaths by forcing them to stay in the ship's broiling, fume-filled hold.

Turkey's official Anatolia news agency meanwhile reported Tuesday that 24 migrants had been rescued after a boat overturned after leaving Turkey's Bodrum peninsula for Kos.

The corpses of five migrants from the boat were found, with a survivor telling AFP the victims had been trapped in the hull.