BRUSSELS (Reuters) - The European Commission warned Greece on Wednesday it could face more border controls with other states of the free-travel Schengen zone in May if it does not fix “serious deficiencies” in its management of the area’s external frontier.

Refugees and migrants arrive aboard the passenger ferry Nissos Rodos at the port of Piraeus, near Athens, Greece, January 27, 2016. REUTERS/Alkis Konstantinidis

EU countries have been increasingly critical of Athens’ handling of the continent’s worst migration crisis since World War Two. More than one million migrants reached Europe last year, mainly through Greece.

“If the necessary action is not being taken and deficiencies persist, there is a possibility to ... allow member states to temporarily close their borders,” European Commission Vice President Valdis Dombrovskis told a news briefing.

He was speaking after the Commission, the EU’s executive body, accepted a report saying cash-strapped Athens had “seriously neglected” its obligations to fellow Schengen states.

The use of that phrase could pave the way for EU governments to exercise the option of reinstalling controls on their national borders for up to two years once short-term measures currently in place expire in May.

Several EU member states have instituted emergency controls on their borders and warned they may effectively suspend Athens from the passport-free zone. Most of the irregular migrants arriving in the EU have come from Turkey via Greece and trekked northward to Germany.

Dombrovskis said Greece was not effectively identifying or registering people arriving, not uploading fingerprinting data to relevant bases systematically and not checking travel documents properly and against key databases.

Greece responded by saying EU interior ministers had concluded at a meeting on Monday that Turkey held the key for tackling the crisis and that it must deliver on agreed measures.

“We think that trying to isolate Greece is not constructive,” said government spokeswoman Olga Gerovassili.

EU border agency Frontex says its mission to the Greek island of Lesbos in January showed improvements in registration procedures. But EU officials carried out an assessment in Greece in November that led to Wednesday’s conclusion alleging “serious deficiencies” in Greek frontier control.

Gerovassili said Greece’s request for support from Frontex had only been partially met and that a plan to relocate refugees, agreed at an EU Council in September, had stalled, with only 414 out of 160,000 refugees relocated.

SEALING GREECE OFF

The step of imposing border controls can be taken for up to six months and can be renewed up to three times for a total of two years.

Dombrovskis said the Commission was intent on preserving Schengen, one of the EU’s key achievements, and said Greece had improved its border controls since November - but not enough.

The next step in the process would be for Schengen member states - 26 countries, most of which are also in the EU - to confirm the Commission’s conclusions in a majority vote. The executive would then recommend remedial measures and assess by May whether Athens had complied.

Greece has no land borders with the rest of the Schengen zone, so installing new frontier checks would affect only air and sea ports.

Diplomats and officials described the move to penalize tourism-dependent Greece as a way to raise pressure on Athens, which is already mired in a financial crisis, to better implement EU measures to deal with the migrant crisis.

The EU is also looking into using Frontex more to help guard the border between Greece and Macedonia, which is not a member of the EU or Schengen. Some Frontex personnel are already at Greece’s northern border, but the agency’s mandate does not allow for interventions in third countries.

Some diplomats said countries could send more police or border guards to Macedonia on the basis of bilateral agreements.

Athens says the influx is impossible to control and its migration minister, Yannis Mouzalas, warned this week that sealing his country off from Schengen would create a humanitarian crisis with thousands of people trapped in Greece.