AL-BAB, Syria/ISTANBUL — When Syrian real estate worker Abu Ahmad was stopped by police in Istanbul he expected a ritual ticking-off for his expired documents, before being allowed on his way. Instead he says he was bundled into a bus packed with 50 men and deported to Syria.

The 31-year-old was stopped as he set off to meet a client in Turkey's bustling commercial hub where up to one million Syrians live — hundreds of whom have been detained this month according to authorities.

His Turkish identity paper, known as a temporary protection permit, was valid for a Turkish province on the Syrian border nearly 1,000 km (625 miles) southeast of Istanbul.

In previous encounters with authorities Abu Ahmad had shown them an expired travel permit allowing him to move around inside Turkey, and escaped with a reprimand.

This time was different for him and dozens of other men who were piled into the bus in Istanbul's Esenyurt district.

Ten days later he says he found himself at the Bab Al-Hawa crossing into Idlib, a northwestern Syrian province controlled by rebels and militants, hundreds of kilometers from his home province of Deir Al-Zor in eastern Syria.

Four others who spoke to Reuters in northern Syria said they had been forcibly sent there in the past week. All had thought they were being transferred within Turkey, not across the border to a country ruined by eight years of civil war.

An employee at the Syrian Bab Al-Hawa crossing said he recorded at least 4,500 Syrian returns this month, but could not say how many were voluntary trips or forced deportations.

The numbers represent only a tiny fraction of the 3.6 million Syrian refugees in Turkey, but the detentions and transfers suggest authorities are stepping up actions to address rumbling grievances over their prolonged presence.

They follow two clashes in Istanbul when crowds attacked Syrian shops, now targets of resentment for Turks who see Syrians as taking jobs and crowding out health and education services while Turkey battles an economic recession.

Most Syrians live in southern Turkish provinces near the border but Istanbul province holds the largest contingent. Many have started hiding at home, waiting for the wave of arrests to recede and some stopping work to express their anger.

Turkey's Interior Ministry says there are 547,000 Syrians registered in Istanbul but the city's new mayor says the total Syrian population may be nearly double that. Surveys conducted for the International Organization for Migration put the total between 600,000 and 900,000, many registered elsewhere.

As the mood sours toward Syrians, the Turkish government has repeated it is working to help Syrians cross back into Turkish-controlled parts of northern Syria. President Tayyip Erdogan said last month that around 330,000 had returned since Turkey launched military operations in Syria three years ago.

The situation in Istanbul gained added prominence ahead of mayoral elections in March and June, when candidates for both main parties said the city was struggling to handle the Syrian influx.

On Monday, Istanbul's governor set a four-week deadline for Syrians without Istanbul permits to return to provinces where they are registered, or face forced removal to those regions.

Interior Minister Suleyman Soylu said that around 1,000 unregistered Syrians had been detained in Istanbul in the last two weeks as part of a wider sweep for illegal migrants, but denied that Turkey was deporting Syrians.

"There are Syrians who are completely unregistered. We take these and send them to camps," he told broadcaster NTV. "We never have deported and cannot deport the Syrians under the scope of temporary protection."

A week after Abu Ahmad was detained, he called his brother Abu Al-Deir to say he expected to be released and was just waiting for the paperwork to be completed.

"Our biggest fear was only him being sent back to Sanliurfa," Abu Al-Deir said, referring to the Turkish border province where he was registered. "We didn't even consider him being deported to Syria." Like his brother, he spoke to Reuters on condition that he not be identified by his full name.

Abu Ahmad said he was taken to a prison near the airport on the Asian side of Istanbul. "The smell was inhumane," he said, describing two inedible meals they were given and the lawyer he said swindled detainees out of hundreds of dollars, promising to get them released. Everyone who paid him was still deported.

A policeman ordered him to sign paperwork in Turkish and Arabic that said he was voluntarily returning to Syria. "I said: 'This is for deportation. I don't want to be deported'."

The policeman told him that the undated document would only be used if he committed a crime. Other policemen came in the room, yelling at and slapping some of the detained Syrians until, Abu Ahmad said, "everyone signed".

Two days later, Abu Al-Deir got another call from his brother, this time from inside Syria. "When he told us at first we were surprised, we thought he was joking," said Abu Al-Deir.

Abu Ahmad's wife, nine months pregnant, and Abu Al-Deir took a 20-hour bus trip from Istanbul to Sanliurfa. Although his wife is in the last stage of gaining Turkish citizenship and his brother has a valid permit to stay in Istanbul, Abu Ahmad said he fears they could both be picked up if they stay in the city, because others with valid papers were deported along with him.

He wants to be smuggled back, but then would have no valid documentation. He also wants to get a lawyer to help sort matters out.

"What makes you angry is, if you'd committed a crime, okay," he said, pausing and taking a sip of tea. "But no one has committed a crime." — Reuters