A militant attack in the mountainous Gilgit-Baltistan region of northern Pakistan, near the Chinese border, has rekindled concerns over security on a highway considered crucial for developing the two countries’ economic relationship.

Over the weekend, a passenger bus traveling on the Karakoram Highway was hit by gunfire near the town of Chilas, less than 400 kilometers from the border. At least 10 people were killed, including two of them Pakistani Army soldiers, and more than 20 wounded.

The attack, of unclear provenance, took place against a backdrop of rising militant activity in the country, involving myriad groups ranging from the Pakistani Taliban to separatists in Balochistan province. But experts see the targeting of the highway as a worry given its envisioned role as a linchpin of Pakistan’s Chinese ties.

The stretch of road, which runs 806 km from the town of Hassan Abdal near Islamabad to the border, was built with Chinese support and opened for traffic in 1986. More recently, the neighbors included upgrades to the highway under the China-Pakistan Economic Corridor (CPEC), the $50 billion Pakistani component of Beijing’s Belt and Road Initiative (BRI).

In October, when Pakistani caretaker Prime Minister Anwaar ul-Haq Kakar visited China, he announced plans to make Karakoram an all-weather road. At present, it is closed from December to April due to heavy snow.

Experts say the highway is essential for plans to connect China with the key southern BRI port of Gwadar in Balochistan.

The Karakoram Highway “forms the most important node of the ambitious CPEC [project],” Khuram Iqbal, a counterterrorism expert at the Department of Security Studies and Criminology at Australia’s Macquarie University, told Nikkei Asia.

But the weekend attack has highlighted yet again the threats that lurk along the way, from the north to the south, where Baloch separatists have been targeting Chinese interests for years in an effort to sabotage the government’s economic ambitions.

Fakhar Kakakhel, an independent analyst specializing in militancy in Pakistan, says the Gwadar to Gilgit route, which he dubbed G2G in a book published this year, is critical to CPEC’s success but is plagued by militants who want to block China’s access to its full benefits. “Militancy in [the Gilgit] region will certainly affect Chinese interests in Pakistan,” Kakakhel said.

Gilgit-Baltistan, with a population of about 1.5 million, was originally a part of Pakistan-administered Kashmir. In 2009, it was granted semi-autonomy and given a chief minister, although it is not a province. Kakakhel said threats to Chinese interests in the area are severe. “Militants from Afghanistan [have] tried to infiltrate Chitral in northern Pakistan, which shares a border with the Gilgit-Baltistan region,” he added.

Iqbal, the counterterrorism expert at Macquarie University, counted as many as 31 armed attacks against Chinese interests and personnel in Pakistan overall. “China has faced the highest number of attacks in Pakistan compared to any other country, including the U.S.,” Iqbal said.

He added that past attacks along the highway had prompted China to recoil from CPEC, and that Beijing was less likely to expand the project “without security guarantees from Pakistan.”

It was only last month that Pakistan expanded security measures for Chinese people working in the country, at Beijing’s request. But the bus attack serves as an unwelcome reminder of the difficulty of guaranteeing safety.

The deadly shooting has disrupted travel along the route. Owners of passenger buses traveling on the Karakoram Highway have suspended services. The government of Gilgit-Baltistan has announced that such buses will only be allowed to cross Chilas during daylight hours, and only in convoys with security escorts.

A security official based in Islamabad, who spoke with Nikkei on condition of anonymity, downplayed the issue, saying there is no direct threat to the Karakoram Highway. “Hostile forces have always tried to exploit such incidents aimed at [stoking] sectarian unrest and relate it with CPEC, which have been thwarted successfully in the past,” the official said.

But experts see those sectarian tensions in the Gilgit-Baltistan region causing problems for Chinese interests in the long run.

“[Gilgit-Baltistan] is a Shia-majority region surrounded by Sunni Muslims and has a long history of Shia-Sunni conflict,” Kakakhel explained, referring to divisions within Islam.

Iqbal said the sectarian divide in the region makes it a hotbed for radicalism and provides ideal conditions for militant groups to establish themselves.

“Religious terrorist outfits [in Gilgit-Baltistan] are going to pose a formidable threat,” he said, “not only to the local communities but also to the international trade passing through [the Karakoram Highway].”