Sana'a, Yemen (CNN) -- Walking past the AK-47s at the gates of Yemen's Central Security Forces seemed rather easy although it was not Yemeni soldiers asking questions -- it was Americans in a Land Cruiser.

The anti-terror headquarters in Sana'a has a number of foreign guests these days, but until now they have never been caught on camera.

Beneath hats and behind sunglasses, U.S. and British military trainers put Yemeni security forces through their paces -- teaching them how to fight al Qaeda.

At the training grounds in the mountains surrounding the ancient city, they were finishing up classes for the day.

Such classes are secretive. They are acknowledged by the governments involved but rarely openly discussed.

The trainers were less than keen on publicity and CNN was ordered to stop filming despite having rarely granted official permission to visit the base. Their numbers have not been confirmed. Over the course of a few days I spotted around ten.

Their responses to a Western journalist ranged from cordial chats to passive aggressive. Most skulked off when they saw me.

Pressure has been on the Yemeni government to fight a growing al Qaeda element -- al Qaeda in the Arabian Peninsular -- which grabbed the attention of the West with the Christmas Day attempted bombing of a Northwest Airlines transatlantic flight as it landed in Detroit. The suspect, Farouk AbdulMutallab, who has pleaded not guilty to six federal terrorism charges, was reportedly trained and armed in Yemen.

There is also increasing scrutiny of America's growing involvement.

They are more than advisers, they hand pick the country's top fighters, said General Yahya Mohammed Abdullah Al Saleh, President Saleh's nephew, who runs the elite counter-terrorism unit (known as the CTU).

"First, when we recruit new blood to the CTU, the British are the first ones to handle the training, and then from there, they pass them to the American trainers, and then more training and only a few can continue to the CTU," said Al Saleh.

"It is financed and supported by the United States Government and the UK."

Although most military officials admit in private they would prefer not to need foreign advice, the general admits it has been useful to his troops.

"Before we used to conduct operations and we [would] find difficulty [in] dealing with terrorists especially as we have [a] lack of training and [a] lack of equipment," he said. "So after the cooperation with the United States we limited our injuries and our casualties."

Each operation is filmed and given to U.S. advisers to analyze, he added.

"We review all operations with the Americans and then we check when we need more training."

But al Qaeda is also stepping up its training in Yemen. Some counter-terrorism experts warn that an influx of foreign fighters from the insurgencies in Afghanistan and Iraq is making the terrorist presence in Yemen much more resilient.

Al Qaeda is using U.S. and British involvement in Yemen as propaganda to win over the support of locals and discredit the Yemeni government.

There is also growing speculation of a more direct role in the fighting by the American military. But U.S. officials maintain they only provide intelligence and training to the Yemenis.

In June, Amnesty International released photographs of U.S. cluster bombs dropped on a rural Yemeni village in an anti-al Qaeda operation. Scores of women and children were reported to have been killed. This attack took place on December 17 -- about a week before the Detroit attempted bombing.

Most Yemeni army officials believe al Qaeda in the Arabian Peninsula numbers only a few hundred, highly-trained fighters living in rural areas where local tribes may provide shelter.

One senior Yemeni government official speaking off the record said that, after the Detroit bomb plot, they came under so much pressure from the U.S. to tackle al Qaeda they moved negotiators out to villages warning them against harboring suspects.

"Yemeni society is not homogenous, there are lots of people who see the Yemeni-U.S. security cooperation as a horrible choice," said Mohammed Al Asaadi, a former editor of the Yemen Observer. "Others believe this kind of cooperation is acceptable as long as it is based on a win-win deal which -- they feel -- is not the case. Whether the U.S. or UK troops are building the capacity of the Yemeni forces or directly are launching air attacks, this kind of military cooperation is publicly unwelcome."

For the Yemeni government, any evidence of foreign involvement in its campaign against al Qaeda risks a backlash. This is one of the most conservative of Arab countries where foreigners are often viewed with suspicion.

The western trainers may play a crucial role in helping confront al Qaeda here -- but in winning the war the government risks losing the hearts and minds of its people.