An armed auxiliary police officer who handled the screening of explosives and dangerous goods at Kuala Lumpur International Airport (KLIA) was among a group of 10 arrested last month for suspected links to Islamic militants, said a senior counter-terrorism source, raising concerns over aviation security in the country.

The suspect is the first auxiliary policeman to be arrested for alleged ties to the Islamic State in Iraq and Syria (ISIS) militant group.

"The auxiliary police officer had been working at KLIA Terminal 1 since 2013. He worked at the scanning machine division," the source told The Straits Times.

Before KLIA, the man worked at Subang airport, the source added.

Describing this as serious, counter-terrorism experts said that as an airport police officer has wide access, he could help smuggle in weapons and militants for terror attacks.

KLIA Terminal 1 serves Malaysia Airlines and all international air carriers while Subang airport mainly serves secondary airlines.

The suspect is currently being held for 60 days under the Prevention of Serious Crime (Amendment and Extension) Act 2013 to help in investigations.

Of the other nine arrested, eight of them, who included five military personnel, have been charged in court.

According to the source, the airport officer allegedly escorted ISIS member Zulqarnain Ghaz Ab Ghani, who is his brother-in-law, through immigration at KLIA when Zulqarnain was on his way to Syria in April last year.

When Zulqarnain returned in August that year, the suspect again escorted him through immigration.

Zulqarnain has been arrested and charged in court.

"As an auxiliary police officer, he had a duty to report his brother-in- law's activities to the police, but he did not. He is believed to be an ISIS sympathiser," said the source, who added that this was a "very serious" issue.

"Such a person could help bring in weapons, explosives, militants to sabotage the airport or an aircraft," the source added.

Counter-terrorism analysts warned that airports are always targeted by terrorist cells and it is crucial that airport screeners themselves are properly vetted.

"Airport auxiliary police screeners need to ensure that no explosive or other hazardous materials can find their way onto commercial aircraft. Hence, they need to be thoroughly security-cleared. That this could happen is certainly cause for serious concern," said Associate Professor Kumar Ramakrishna, head of policy studies at the S. Rajaratnam School of International Studies.

"We should bear in mind that since September 2014, ISIS has, through its online forums, called on its sympathisers to strike at unbelievers anywhere in the world through any means," he said.

"This could take the form of lone wolf attacks... especially if radicalised auxiliary policemen are armed.

"Alternatively, the sympathisers among airport security personnel could facilitate attacks by other lone wolves and wolf packs acting on their own but ideologically inspired by extremist ideologies; or even attacks by organised and centrally directed terrorist cells," he added.

The counter-terrorism source said the police have met Malaysia Airports Holdings Berhad (MAHB) to advise the airport operator to tighten its background checks on airport security employees.

When contacted, an MAHB spokesman told The Straits Times that its staff are vetted by the police. "All background checks on our airport security personnel are conducted by the police," Tuan Mustafa Kamal Haji Alang Othman, MAHB's general manager of aviation security, said.