Beijing fears these fighters — instrumental in ground offensives against Assad’s forces — may one day return to foment terror.

Many don’t speak Arabic and their role in Syria is little known to the outside world, but the Chinese fighters of the Turkistan Islamic Party (TIP) in the Arabic nation also known as the East Turkistan Islamic Movement (ETIM) are organised, battled-hardened and have been instrumental in ground offensives against President Bashar Assad’s forces in the country’s northern regions.

Thousands of Chinese Jihadis have come to Syria since the country’s civil war began in March 2011 to fight against government forces and their allies. Some have joined the al-Qaeda’s branch in the country previously known as Nusra Front. Others paid allegiance to the Islamic State group and a smaller number joined factions such as the ultraconservative Ahrar al-Sham.

But the majority of Chinese Jihadis are with the TIP in Syria, whose vast majority are Chinese Muslims, particularly those from the Turkic-speaking Uighur majority native to Xinjiang in China. Their growing role in Syria has resulted in increased cooperation between Syrian and Chinese intelligence agencies, who fear those same Jihadis could one day return home and cause trouble there.

The ETIM considers China’s Xinjiang to be East Turkistan.

Their aim: remove Assad

Like most Jihadi groups in Syria, their aim is to remove Mr. Assad’s secular government from power and replace it with strict Islamic rule. Their participation in the war, which has left nearly 4,00,000 people dead, comes at a time when the Chinese government is one of Mr. Assad’s strongest international backers. Along with Russia, China has used its veto power at the United Nations Security Council on several occasions to prevent the imposition of international sanctions against its Arab ally.

Beijing has blamed violence back at home and against Chinese targets around the world on Islamic militants with foreign connections seeking an independent state in Xinjiang. The government says some of them are fleeing the country to join the Jihad, although critics say the Uighurs are discriminated against and economically marginalised in their homeland and are merely seeking to escape repressive rule by the majority Han Chinese.

Abu Dardaa al-Shami, a member of the now-defunct extremist Jund al-Aqsa group, said the TIP has the best “Inghemasiyoun,” Arabic for “those who immerse themselves.” The Inghemasiyoun have been used by extremist groups such as the IS and al-Qaeda’s ffiliate now known as Fatah al-Sham Front. Their role is to infiltrate their targets, unleash mayhem and fight to the death before a major ground offensive begins.

‘Lions of ground offensives’

“They are the lions of ground offensives,” said al-Shami, who fought on several occasions alongside TIP fighters in northern Syria.

Xie Xiaoyuan, China’s envoy to Syria, told reporters in November that the two countries have had normal military exchanges focused on humanitarian issues, although Chinese officials have repeatedly rejected the possibility of sending troops or weapons.

In the last year, however, Chinese and Syrian officials have begun holding regular, once-a-month high-level meetings to share intelligence on militant movements in Syria, according to a person familiar with the matter.

“These people not only fight alongside international terrorist forces in Syria, but also they will possibly return to China posing threat to China’s national security,” said Li Wei, terrorism expert at China Institutes of Contemporary International Relations and Director of the CICIR Institute of Security and Arms Control Studies.

5,000 Chinese fighters?

Rami Abdurrahman who heads the Britain-based Syrian Observatory for Human Rights, said there are about 5,000 Chinese fighters in Syria, most of them with the TIP fighters in northern Syria who along with their families make about 20,000. Mr. Li has said Mr. Abdurrahman’s numbers are way too high, adding that he believes the number are about 300 Chinese fighters in Syria who brought with them about 700 family members.

“As the control of the passage along the borders between Turkey and Syria is being tightened, it is becoming more difficult for them to smuggle into Syria,” Mr. Li said.

Syrian opposition activists and pro-government media outlets have said dozens of TIP fighters have carried out suicide attacks against government forces and their allies and for the past two years have led battles mostly in the north of the country.

The suicide attackers include one known as Shahid Allah al-Turkistani. He was shown in a video released by TIP taken from a drone of an attack in which he blew himself up in the vehicle he was driving near Aleppo late last year, allegedly killing dozens of pro-government gunmen.

In 2015, members of the group spearheaded an attack on the north-western province of Idlib and captured the strategic town of Jisr al-Shughour on the edge of Mr. Assad’s stronghold of Latakia region. They reportedly damaged a church in the town and raised their black flag on top of it.

In late 2016, TIP was a main force to briefly break a government siege on the then rebel-held eastern parts of the northern city of Aleppo.

They know more than others: Assad

The role of the Chinese Jihadis in Syria was a topic that Mr. Assad spoke about last month in an interview with Chinese PHOENIX TV, saying “they know your country more than the others, so they can do more harm in your country than others.”

Unlike other rebel groups, TIP is a very secretive organization and they live among themselves, according to activists in northern Syria. They are active in parts of Idlib and in the strategic town of Jisr al-Shughour, as well as the Kurdish Mountains in the western province of Latakia.

Abdul-Hakim Ramadan, a doctor who was active in Idlib province, said one of his teams was trying to enter a north-western village to vaccinate children when TIP fighters prevented them from entering, saying only Chinese can go into the area.

Mr. Ramadan said unlike other fighters who have come to Syria, the Chinese have not merged into local communities and the language has been a major barrier.