Turkey has already agreed to take back refugees who have already used it as a transit country to enter Europe, in exchange for the visa-free deal, but the EU believes Ankara will not be able to implement reforms on freedom of the press and the judiciary by June 30.

Turkey's 75 million citizens would have the right to enter the Schengen zone for up to 90 days at a time with biometric passports from the end of June.

The EU has a list of 72 requirements that Ankara needs to meet to obtain visa-free travel, with reform of anti-terror legislation another of the five remaining key steps, along with the protection of personal data.

"The questions I had in this connection have not been fully cleared up," Merkel said.

Terrorists would be more likely to attack EU countries as a result of the deal to allow Turkish citizens to travel across the continent without visas, EU leaders said last week.

"Foreign terrorists and organized criminals are 'expected' to seek Turkish passports to reach continental Europe 'as soon as' the visa waiver program comes into force," a European Commission report said.

Merkel's concerns

Meeting with Turkish President Recep Tayyip Erdogan Monday, Merkel said she had "made it very clear" that the move to strip about 25 percent of Turkish members of parliament (MPs) - many of whom are from the Kurdish minority - of their legal immunity as "a reason for deep concern."

UN on the job

World leaders and aid groups met at an unprecedented aid summit in Istanbul, headed by UN Secretary General Ban Ki-moon. At the event Erdogan stressed Turkey's contributions in hosting three million refugees from the Syria and Iraq conflicts.

"The current system falls short... the burden is shouldered only by certain countries, everyone should assume responsibility from now on," he said. "Needs increase every day but resources do not increase at the same pace. There are tendencies to avoid responsibility among the international community." He added that Turkey had spent $10 billion on hosting Syrian refugees, compared to $450 million from the rest of the international community.

jbh (dpa, AFP)