Afghan police arrested two British private security contractors and two Afghan colleagues and ordered their company closed down after finding a cache of weapons in their vehicle, an official said Thursday. They are being held for investigation into illegal arms transport.

Their detention spells the latest trouble for Afghanistan's dozens of private security companies that guard supply convoys, development projects and private businesses. President Hamid Karzai has ordered all the protection companies shut down by March, to be replaced by a unified government-run protection force.

Police who stopped the contractors' vehicle at a Kabul checkpoint Tuesday found more than two dozen AK-47 rifles in a metal box covered by a blanket, Ministry of Interior spokesman Sediq Sediqi told a press briefing.

All 30 weapons had their serial numbers scratched off, and the men had no permits for them, so police arrested all four men on suspicion of illegal arms transport, Sediqi said. He said the case has been sent to Afghanistan's attorney general for investigation.

Authorities ordered the immediate shutdown of Afghanistan operations of their company, the international security consulting firm GardaWorld, and are questioning other company employees.

"They have to pay all the dues they owe to the government of Afghanistan, and they cannot operate any more after that," Sediqi said.

GardaWorld specializes in high-risk areas around the world, with offices in Iraq, Pakistan, Yemen and Haiti. It provided security for Afghanistan's 2005 National Assembly elections.

The firm said Thursday that it was cooperating with the Afghan investigation. A statement indicated it did not own the AK-47s but was in the process of buying them through legal channels.

"The weapons in question were being taken to be tested at a firing range before being purchased and properly licensed by GardaWorld," the company said. "We fully comply with all laws and regulations in our Afghanistan operations and are making every effort to work closely with the Afghan authorities to rectify the situation as soon as possible."

A spokesman for the British Embassy said it was monitoring the case and providing consular services to the two British citizens.

Afghanistan has been scrambling to train guards for its own government security service _ called the Afghan Public Protection Force, or APPF _ since Karzai late last year ordered all 103 private security companies closed by March 2012.

Karzai has said the private security firms undermine the Afghan police and army forces, creating effective militias that often flout Afghan laws and regulations.

Controversies caused by some contractors' behavior, ranging from violence to cultural insensitivity, has given the industry a bad name among many Afghans.

In 2008, guards from the American security giant Blackwater Worldwide _ now known as Xe Services _ forced an Afghan soldier to the ground and handcuffed him after he refused to let their vehicle pass through a checkpoint.

And in 2005, three DynCorp International guards assigned to Karzai's own protective detail caused a scene in the VIP lounge of the Kabul airport while awaiting a flight. "They had been intoxicated, loud and obnoxious," according to an internal company report. DynCorp, also a U.S. firm, fired the three guards.

So far, 57 of the private security companies have been shut down in Afghanistan, Sediqi said Thursday. Another 46, half of them Afghan firms and half international, are still operating but officials have vowed to close them by March, according to the Interior Ministry.

The new Afghan force will need to train 25,000 guards to take over all the work performed by privately contracted guards, according to a U.S. government report released in October.

Recruitment has been slow. As of late last year, the APPF had only about 6,500 guards trained, the U.S. report said. The NATO force in Afghanistan has offered help to speed up the training of the Afghan guards.

If the Afghan government is not ready to take over by March, there is a provision for 12-month extension that would allow the private firms to continue operating. Karzai originally set a deadline to shut down private contractors by the end of 2010, but it was pushed back to this year.