Colombia launched a major military and police operation Thursday to try to rescue four Chinese oil workers abducted by suspected leftist guerrillas in a remote southern jungle region.

A land and air operation was under way around the town of San Vicente del Caguan, a longtime stronghold of the leftist Revolutionary Armed Forces of Colombia (FARC) guerrillas that government forces retook in 2002.

The Chinese employees -- three engineers and a translator working for the British oil company Emerald Energy, a subsidary of the Chinese company Sinochem -- were taken hostage on Wednesday after being seized from their vehicle by seven guerrillas, thought to be FARC.

The kidnappers released the Colombian driver, who informed company officials and police two hours later.

"We are making a serious effort to try to generate pressure for the hostages to be released," top regional official Edilberto Ramon Endo told reporters.

But a defense department official, speaking off the record, said it would be a tall order to find the men, even with the massive military effort.

"They're lost. There are no clues, no-one has seen them," the source told AFP, saying the region was a sort of Colombian badlands where the government only has limited control.

"This is the area where the FARC commits their crimes," he said. "It's clear that they (the hostages) are there."

Endo believed the abductions were part of a racketeering campaign by the guerrillas to try to shake down Emerald Energy for protection money.

Military sources said they believe the abduction was carried out by an elite FARC unit -- the Teofilo Forero group -- which President Juan Manuel Santos has ordered the military to step up actions against.

Colombia meanwhile said it has deployed its elite Omega joint task force -- comprised of army, air force and marine units which operates mostly in the south of the country -- to search for the abducted men.

The air force was conducting flights over the area where the men are believed to be held, using combat, intelligence and reconnaissance aircraft, while army troops and national police troops scoured the region.

Colombian army General Javier Florez told RCN radio in Bogota that the armed abductors were dressed in civilian clothes.

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. Several other guerrilla groups, however, operate in the country.

Last year, 282 people were kidnapped in Colombia, 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 foreign multinationals would be kicked out if they paid ransoms to guerrillas or criminal gangs for protection or to ransom kidnapped workers.

That warning, in March, came after more than 20 employees of South American Exploration, a subcontractor of Canadian oil company Talisman Energy and Colombia's Ecopetrol, were freed by their kidnappers, with Santos noting there was talk among the freed hostages of a ransom of some $2.6 million.