BRUSSELS (Reuters) - More Afghans than Syrians have migrated to the European Union so far this year, official data shows, making them the largest nationality illegally entering the bloc, with many relocating from Iran partly due to the hardship caused by U.S. sanctions.

While the EU fears a new wave of asylum seekers from Syria following Turkey’s attack this week on the Kurdish-controlled northeastern region of the country, EU officials warned that the increasing number of Afghans posed a more immediate problem, especially in the Greek islands where many of them first arrive.

Since the beginning of the year, nearly 17,000 Afghans crossed the Aegean sea to reach EU shores, the latest data from the EU border agency Frontex show.

Around half of them had been living in Iran before attempting the crossing through Turkey into the EU, a senior EU official told Reuters, adding that in many cases Afghan migrants arriving to Europe were born in Iran.

They are leaving because “the U.S. policy has significantly deteriorated the economic situation in Iran,” the official said.

Iran’s economy has been squeezed since President Donald Trump reimposed sanctions last year to pressure Tehran to give up what Washington calls its pursuit of nuclear weapons.

Facing hard times in neighboring Iran, Afghan workers are leaving to seek better ways to support families left behind in Afghanistan, Iranian officials said.

The situation in Afghanistan remains fraught with risk, as Taliban militants presently control more territory than at any time since they were ousted from power by U.S.-led forces nearly two decades ago.

There are currently about 2 million Afghans in Iran, Afghan officials said. Nearly 800,000 left last year according to the International Organisation for Migration.

Only a very few of them went to Europe in 2018 when about 12,000 Afghans were recorded to have illegally crossed the EU borders. But that number could now double.

Most of them have landed on Greek islands, where authorities are struggling to provide shelter. Charity group Oxfam said over 13,000 men, women and children were now crammed into the Moria camp on Lesbos, which was designed to accommodate 3,100 people.

EU officials stressed that the situation remained manageable and was nowhere near as critical as in 2015, when more than 1 million migrants reached the bloc - 25% from Afghanistan - causing the worst migration crisis Europe had seen since World War II.

Still, one EU official said the situation on the Greek islands was becoming “disturbing” once again, while German Interior Home Minister Horst Seehofer warned last week that the EU risked a repeat of the 2015 chaos if member states did not adopt a common policy on migration.

In total, around 80,000 migrants have reached the EU in the first nine months of this year, but numbers are growing and the declining trend seen since the peak year in 2015 could be broken.

Syrians seeking asylum in the EU have dropped to about 10,000 so far this year from more than half a million in 2015, but they are once again fleeing their homes.

More than 130,000 people have been displaced in northeast Syrian since the beginning of a Turkish offensive last week, United Nations said.

Turkish President Tayyip Erdogan has threatened to send more than 3 million Syrian refugees sheltered in Turkey to Europe if the EU opposed Turkey’s actions in northern Syria.