The German chancellor has sought the creation of "safe zones" to shelter refugees inside Syria, a proposal criticised by the UN and rights groups.

Speaking at a university in the city of Gaziantep, Angela Merkel called for "zones where the ceasefire is particularly enforced and where a significant level of security can be guaranteed".

Keeping people displaced from their homes on the Syrian side of the border would help the 28-nation bloc and Turkey, which hosts 2.7 million Syrian refugees, stem the flow of refugees to European shores.

Merkel, who visited Nizip refugee camp near the Syria border along with senior EU leaders on Saturday, heaped praise on Turkey for its efforts at the camp.

"Our goal is not only to stop illegal migration, but for refugees to have more opportunities near their home," Merkel said.

The UN has warned against the safe-zone plan unless there is a way to guarantee the refugees' safety in the war-torn state. Aid workers have also opposed it.

On Sunday, US President Barack Obama said during a news conference with Merkel that he was not supporting the idea.

"Sadly, it is very difficult to see how it would operate short of us essentially being willing to militarily take over a big chunk of that country," he said.

IN PICTURES: Syrian refugees mass at Turkish border

A ceasefire between the Syrian government and rebel groups in place since February end has faltered, forcing tens of thousands of people to flee the fighting.

But they are not allowed to cross into Turkey, and instead are camped near the Azaz border crossing, where local agencies offer humanitarian support.

Turkey only allows critically injured Syrians to enter the country where more than 2.7 million refugees are registered.

EU-Turkey deal

EU leaders have faced criticism for striking a deal last month with Turkey under which Ankara would get a package of incentives - from billions in refugee aid to progress on visa-free access to the bloc for Turkish citizens - in exchange for help in returning refugees.

The deal to send back thousands of refugees and migrants from the Greek islands to Turkey has been dubbed as immoral and a violation of international humanitarian law. Rights groups say Turkey is not a country where returnees can be guaranteed proper protection.

OPINION: The dark side of the EU-Turkey refugee deal

Turkish Prime Minister Ahmet Davutoglu said the deal had already had a significant impact and that the number of people illegally crossing the Aegean Sea each day had dropped from 6,000 in November to 130.

One side of the bargain, used to sell the refugee deal to the Turkish public, was Turks' winning quicker visa-free travel to Europe, a pledge that now could go unfulfilled, at least by the June deadline that Prime Minister Davutoglu had wanted.

On Saturday Davutoglu said there would be no more re-admissions if visa liberalisation was not enacted, but that he believed the EU would take the necessary steps.

Davutoglu, Merkel, EU Council President Donald Tusk and Vice President of the European Commission Frans Timmermans visited the Nizip camp on Saturday and inaugurated a child protection centre in Gaziantep.

Hundreds of thousands of Syrian children in Turkey still have no access to education.

Davutoglu said Turkey had met all its responsibilities, including giving refugees the right to work. But a work permit scheme for refugees designed to protect them from exploitation has been slow to gain traction.

Yet Tusk on Saturday praised Turkey as a refugee host.

"Today Turkey is the best example in the entire world of how to treat refugees. I am proud that we are partners. There is no other way," he said.

Amnesty International has said Syrians are being shot at trying to enter Turkey while others are being deported to Syria against their will, a claim Davutoglu refuted on Saturday.