The Turkish opposition accuses Ankara of turning a blind eye as Islamist militants flood across the border to join Syria's war.
A rebel fighter falls in Aleppo - but this one was from Istanbul
ISTANBUL // Osman Karahan, an Istanbul lawyer with radical Islamist views, told colleagues he was travelling to Iskenderun near the Syrian border to attend a trial. In fact, he crossed into Syria to join the fight to topple Bashar Al Assad.
The lawyer was shot and killed by regime forces in Aleppo on Saturday. He was buried by fellow fighters in Syria, but a vigil for him is planned in an Istanbul mosque after Friday prayers today.
"He has become a martyr, God willing," said Yavuz Cengiz, a colleague of Mr Karahan in Istanbul.
Opposition politicians from Turkey's border region say the lawyer was one of several hundred non-Syrian fighters, many of them Islamist militants, who entered Syria via Turkey in recent months.
They accuse the government in Ankara of turning a blind eye to the militants and to arms shipments for Syrian rebels, with weapons and ammunition sometimes smuggled in Turkish ambulances.
A member of the Syrian opposition in exile in Istanbul said he had no information about a widespread influx of foreign fighters into Syria.
"There may be some isolated cases," said Mahmut Osman, Turkey representative of the Syrian National Council. "The Free Syrian Army does not need fighters anyway, they need weapons and ammunition."
But one expert in Turkey said some radical Islamist groups regarded the conflict in Syria as a "holy war" because an Alawite elite was fighting to keep power over a mostly Sunni population. He said several hundred militants from Turkey alone had joined the fight in Syria.
The use of Turkish territory as a launch pad for foreign Islamists on their way to Syria would be hugely embarrassing for the government, given Turkey's calls for an end to the violence in Syria and concerns among Turkey's western allies about activities of militant groups such as Al Qaeda in Syria.
Recep Tayyip Erdogan, the Turkish prime minister, openly supports the political opposition against Mr Al Assad and has been calling on the Syrian leader to resign. But Turkey insists it does not send arms or fighters over the 900-kilometre border.
But the opposition in Ankara says that does not cover the activities of foreign militants. "They move around in cars and buses," said Mehmet Ali Ediboglu of the opposition Republican People's Party, the CHP. "There are hundreds, if not thousands. They come from places like Libya, Chechnya, Afghanistan and Africa."
Mr Ediboglu and Mevlut Dudu, another CHP politician, said foreigners were renting houses near the border to shelter foreign fighters before and after they take part in clashes in Syria. Mr Dudu said Turkish ambulances carried weapons and ammunition into Syria and returned with wounded fighters for treatment in Turkish hospitals.
Mr Karahan, the Istanbul lawyer, was known in Turkey as the legal representative of several high-profile Islamists, among them Louai Sakka, a Syrian said to be a member of Al Qaeda.
In 2007, Sakka was sentenced to life in prison for masterminding a series of lorry-bomb attacks on synagogues and British interests in Istanbul in 2003, in which 57 people were killed. A partial retrial, ordered by Turkey's court of appeals, is continuing, but Sakka is still in prison. Mr Karahan also defended other Islamists in court.
Mr Cengiz said his colleague was killed during a fire fight for the control of a police station in Aleppo.
Mr Karahan's family said he had dedicated his life to "Muslims under persecution in the world and in Turkey", and the armed resistance against Syrian forces was a "holy fight".
There are no official figures about how many foreigners from Turkey and other nations have joined the Syrian rebels, but Veysel Ayhan, chairman of the International Middle East Peace Research Centre, a think tank in Ankara, said there were more than just a few individuals.
"We're not talking about one or two people." More fighting in Syria could attract even more, he said.
Mr Ediboglu of the CHP said the Erdogan government remained passive to the developments because they were in line with Ankara's stance in Syria. "Turkey is a party to the conflict there," he said. "Erdogan has called Syria an enemy state."
But Mr Erdogan's policy carried the risk of widening the conflict, amid concerns that Syria could encourage Kurdish rebels to increase their attacks in Turkey, Mr Ediboglu said.
"We are meddling there, and now they have started meddling here," he said.