Cologne (Germany) (AFP) - A German mayoral candidate active in helping refugees was seriously wounded on Saturday in what police described as a stabbing with a "racist, political" motive, heaping further pressure on Chancellor Angela Merkel over the migrant crisis.

The German leader expressed her "shock" over the attack in the western city of Cologne which left mayoral hopeful Henriette Reker with serious neck wounds.

The stabbing comes as Merkel prepares to travel to Istanbul on Sunday for talks on the crisis with Turkish President Recep Tayyip Erdogan, who has heaped scorn on Europe's efforts to deal with the problem so far.

She has pushed for a fairer distribution of people across the European Union, but the crisis has sorely tested ties between member states and put unprecedented strain on the right to free movement that is at the core of the 28-nation bloc's values.

Slovenia said Saturday it was drafting in the army to help police cope with an expected influx of thousands of migrants arriving after Hungary shut its border with Croatia overnight in its latest hardline move to stop the flow of people.

More than 630,000 people fleeing war and misery in the Middle East and Africa have landed on Europe's shores this year, with many making risky sea crossings from Turkey to Greece.

Highlighting the human cost of the crisis, another 12 people drowned on Saturday when their wooden boat sank off the Turkish coast as they were trying to reach the Greek island of Lesbos, Turkey's Anatolia news agency reported.

Most of the refugees and migrants are trying to get to Germany, Europe's economic powerhouse, which has said it expects up to one million asylum seekers this year after saying it would open its doors to Syrian refugees.

Merkel's policy has led to growing tensions in Germany, triggering a backlash from her conservative allies and spawning a growing number of increasingly vocal far-right protests.

Reker, an independent close to Merkel's ruling Christian Democrats (CDU) responsible for refugee issues in Cologne, was stabbed in the neck at a party information stand in the city.

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Four other people were also injured, one of them seriously, with regional police chief Wolfgang Albers saying it was a "political act".

The attacker was arrested at the scene and told police he had "a racist motivation for committing this act," Cologne police official Norbert Wagner said at a news conference.

- Slovenia calls in troops -

The flood of people seeking to reach northern Europe showed no signs of abating, with thousands of migrants seeking a new route through Slovenia after Hungary's overnight border closure.

Croatia began ferrying migrants by bus and train to Slovenia on Saturday, away from the Hungarian frontier, and the Slovenian government said 2,700 men, women and children had entered its territory by late Saturday.

The tiny Alpine state says it can handle up to 8,000 refugees a day crossing through on their way to Austria, which has also reinforced its border ahead of the anticipated rise in the numbers.

Slovenian Prime Minister Miro Cerar has sent troops to the border with Croatia but said they would simply help police with logistics and equipment, insisting the move "does not mean there is an extraordinary situation".

Hungary's border closure came as much-hyped EU attempts to reach a deal with Turkey ran into difficulties, with Erdogan on Friday ridiculing the bloc's efforts to help Syrian refugees, and urging Brussels to take Ankara's EU membership bid more seriously.

"They announce they'll take in 30,000 to 40,000 refugees and then they are nominated for the Nobel for that. We are hosting two and a half million refugees but nobody cares," Erdogan said.

The comment appeared to be a jab at Merkel who had been tipped as a Nobel Peace Prize contender earlier this month, in part over her welcoming stance to the refugees.

Turkey's demands in return for helping to stem the migrant flow into Europe include three billion euros ($3.4 billion) in aid and an easing of visa restrictions.

Meanwhile at the Slovenian border town of Petisovci, a large white tent was set up to allow the authorities to carry out identity checks before taking the migrants to one of several refugee camps.

Caroline van Buren, a spokeswoman for the UN refugee agency, said procedures at the Petisovci checkpoint were running "really smoothly".

"Unlike other countries, Slovenia had time to prepare... It's not perfect, but things are moving," she said, adding that most of the arrivals were from Syria, Iraq and Afghanistan.