More than 20 Chinese road builders have been captured by rebels in Sudan, highlighting the growing risks faced by Chinese companies that have poured billions of dollars and thousands of workers into some of Africa’s most volatile areas.

The attack occurred Saturday in Sudan’s South Kordofan — a region rich in oil that abuts the newly established South Sudan— and escalated tensions between the two countries, one largely Muslim, the other dominated by Christians. China has extensive interests, mostly in petroleum production and construction, on both sides of the ill-defined border.

The Foreign Ministry in Beijing said Sunday that Sudan and China had “initiated an emergency response” but did not elaborate, saying only that Sudanese authorities were trying to “locate and rescue the missing Chinese nationals.”

The rebel raid in Sudan coincided with a visit to the region by a senior Chinese official and the opening in Addis Ababa, the capital of neighboring Ethiopia, of the new headquarters of the African Union— a $200 million office tower and conference center paid for and built by China. The building, the tallest in the city, is meant to showcase the benefits of China’s expanding presence in Africa.

China’s official Xinhua News Agency reported Sunday that more than 20 Chinese workers involved in road construction in Sudan had “gone missing” during a raid by militants the previous day. It said they worked for Sinohydro, a hydropower engineering and construction company. The company had no immediate comment.

Human rights groups have accused Sudan of stepping up road building in South Kordofan to bring more troops into the region to crush rebels. The rebel Sudan People’s Liberation Movement-North claimed responsibility for Saturday’s attack and said 29 Chinese workers had been captured, along with a number of Sudanese soldiers. The group has close ties with South Sudan’s ruling party, the Sudan People’s Liberation Movement, which began as a rebel outfit and led the south’s struggle for independence from Khartoum.

Chinese workers have been caught before in fighting between Sudanese troops and rebels, but this is the first time so many have been captured. It is also the first time Chinese workers have been targeted since South Sudan broke away from the north last July. A rebel spokesman told the news agency Agence France-Presse that all those captured were safe.

China has long had close ties with Sudan, providing arms and diplomatic cover to a regime that is ostracized by the West. The China National Petroleum Co., or CNPC, dominates a group of foreign consortiums that won oil concessions in the 1990s. Most of these oil fields, however, now lie in the new state of South Sudan and in increasingly violent border areas.

Feuding between the north and south over the division of revenue has played havoc with China’s oil interests. South Sudan recently shut down production at fields operated by CNPC in protest of the seizure of oil shipments by Khartoum. Most of the oil is produced in the south, but the only way to get it to market is through a pipeline that passes through the north.

At a meeting Saturday in the Ethiopian capital ahead of an African summit, Sudan’s president, Omar Hassan al-Bashir, an indicted war crimes suspect, discussed the oil crisis with Jia Qinglin, the chairman of the Chinese People’s Political Consultative Conference, and urged China to use its influence to break the impasse. He also said Chinese companies working in Sudan “will be protected by concrete measures.”

Jia hailed the two countries’ “traditional friendship” and said that “they have always trusted and understood each other despite changing international landscape and situations of both countries,” according to the Xinhua News Agency.

China has nonetheless grown increasingly frustrated by a crisis that jeopardizes billions of dollars in investment, as well as the lives of Chinese workers. Beijing in December sent its special envoy for Africa to the region to push for a settlement, but the effort failed.