Santos said a reward of up to 500 million pesos (about 285,000 dollars) was available for helping land the arrest of the rebel known as Claudia, with the Marxist Revolutionary Armed Forces of Colombia (FARC). Colombian authorities blame her for the abductions.

The four men, who are employees of the British oil company Emerald Energy a subsidiary of Sinochem, were kidnapped by at least seven armed men wearing civilian clothes June 9.

They were traveling in a region where the FARC are especially active, near the town of San Vicente del Caguan, in the southern Caqueta region.

"Claudia," whose real name is Maria del Carmen Perez, 35, has been a rebel for 19 years. Authorities say she is a leader of an elite FARC unit.

The FARC, which has been at war with the Colombian government since 1964, is the country's oldest and largest leftist group, with an estimated 8,000 combatants.

Last year, 282 people were kidnapped in the South American country -- a 32 percent increase compared to the previous year, according to official figures.

Criminals were responsible for most of the kidnappings -- 57 percent -- compared to 35 percent for leftist guerrillas.

Earlier this year, Santos warned multinational companies they would be kicked out of the country if they paid ransoms to rebel groups or other criminal gangs that have kidnapped their employees.

Around 50 demonstrators, greatly outnumbered by the security forces, were stopped and forced to disperse after they gathered close to the Chinese embassy in the capital Hanoi.

"Down with China! Down with arresting patriotic people!" shouted the protesters as they waved banners denouncing Chinese "violations" of Vietnamese sovereignty.

It was the seventh in an unprecedented series of protests -- uncommon in authoritarian Vietnam -- that have taken place in Hanoi on recent weekends during an escalating maritime dispute in the South China Sea.

The two communist neighbours have long been at odds over the potentially oil-rich Paracel and Spratly island groups, which straddle vital commercial shipping lanes in the South China Sea.

Authorities in Hanoi allowed the first five protests to proceed without incident, but briefly detained 10 people, including journalists, during the rally last Sunday after talks with China in Beijing on June 25.

At the meeting, both sides agreed to resolve their territorial disputes peacefully. Beijing and Hanoi "also laid stress on the need to steer public opinion in the correct direction", the official Vietnam News said.

Tensions flared in May when Vietnam said Chinese marine surveillance vessels cut the exploration cables of an oil survey ship and Hanoi has accused its neighbour of harassing Vietnamese fishing boats in the disputed waters.

On Friday Vietnam and the United States began a joint naval drill in the South China Sea, despite Chinese objections.