Three US contractors were shot dead in an insider attack at the international airport in Kabul, Afghanistan, on 29 January.

Besides the three contractors working for US-led forces in Afghanistan, the Wall Street Journal reported that an Afghan national was also killed in the shooting, which took place in late afternoon at the military side of the airport.

US military officials said that the attack at approximately 6:40pm may have involved an Afghan security officer as it was found that the attacker was wearing the uniform of an Afghan national policeman.

"An investigation into the attack is currently underway."

However, it is not clear if the attacker was actually a member of Afghanistan’s security forces.

An investigation into the attack is currently underway.

In April 2011, the Kabul airport witnessed an Afghan air force pilot killing eight US Air Force officers and a military contractor.

Approximately 13,500 US and allied troops are currently in Afghanistan to train Afghan forces, who are now fighting the Taliban insurgency, the journal said.

Nearly 21,000 contractors have been appointed to handle training and other tasks.

Peaking in 2012, green-on-blue attacks have been a major problem for the US-led coalition in recent years.

On Thursday, a suicide attacker blew himself up in eastern Laghman province, killing 16 people and injuring almost 40.