Abductions in Sudan and Egypt highlight the dangers facing China's workers abroad and led Beijing to reassess a long-stated policy of non-interference as its foreign interests grow, analysts say.

China has been sending workers to Africa since the 1950s to build roads and railways, but investment has surged in the past 15 years as the Asian powerhouse sought to secure the resources it needs to fuel its booming economy.

Trade between Africa and China topped $120 billion last year, a jump from less than $20 billion a decade earlier, and experts say China's interests on the continent are shifting to investing in institutions and governments.

Jonathan Holslag, a researcher at the Brussels Institute of Contemporary Chinese Studies, said the presence of Chinese workers in some of the world's most unstable areas was making them a target for criminals.

"Armed gangs increasingly consider Chinese labourers an easy target for ransom," said Holslag. "Especially in Africa, China's growing economic presence has stumbled into growing insecurity."

He made the comments after the abduction late last month of 29 Chinese workers by rebels in Sudan's South Kordofan state.

That was followed on Tuesday by the capture of 25 Chinese workers in Egypt by Bedouins demanding the release of relatives imprisoned by the fallen regime of Hosni Mubarak. The Chinese were soon freed.

Chinese workers have been killed in Sudan, Ethiopia, Kenya and Nigeria, while others have had to be evacuated when violence has broken out.

Last year, China conducted its biggest and most expensive rescue mission when it evacuated 36,000 of its nationals from Libya, where it has substantial oil interests, as the country descended into civil war.

The violence led to heavy losses for Chinese businesses, which had around $18.8 billion worth of contracts in Libya.

Even in times of peace, there are often tensions between Chinese companies and local people angered by the influx of Chinese workers and allegations that the firms mistreat locally hired staff.

In Zambia, such allegations of mistreatment became one of the campaign themes of Michael Sata, who was elected president in September.

Chinese workers have also been the victims of Beijing's support for regimes shunned by the rest of the international community.

Experts say rebel groups have sought to punish China, whose proclaimed policy of non-interference in the affairs of other countries has led to accusations it props up unsavoury governments.

But Christopher Alden, a specialist in Sino-African affairs at the London School of Economics, said Beijing had been increasingly willing to step in as it seeks to protect its overseas interests.

"They hold to the principle (of non-interference) rhetorically and they will continue to do so, but their actions for the last four or five years have indicated they are willing to bend the principle for a variety of reasons," he told AFP.

In the past, China has sometimes turned to international or regional organisations, working with the International Committee of the Red Cross in 2007 to negotiate the freeing of seven hostages held in Ethiopia.

It has also sought to limit risks by building alliances in the regions where it invests, for example cultivating good relations with the new government of oil-rich South Sudan as well as with Khartoum.

But if that fails, as it did in Libya, they must take on the task themselves, according to Jonas Parello-Plesner, researcher at the European Council on Foreign Relations.

After the latest abductions, many Chinese people took to microblogs to demand action and experts say the government needs to demonstrate that it is looking after China's interests abroad.

"A honeymoon decade of frictionless business expansion worldwide is over," Parello-Plesner wrote in the Financial Times.