Even as the overall number of immigration offenders dipped to a 13-year low last year, the number of harbourers and employers of these offenders arrested rose sharply.

The Immigration and Checkpoints Authority (ICA) said in its annual statistics report released on Wednesday that 2,530 such offenders were nabbed last year - a 2 per cent drop from 2,580 in 2012.

Among them were 600 illegal immigrants - down from 690 in 2012 - and 1,930 overstayers who did not have the proper papers, up from 1,890 in 2012.

But the number of people arrested for harbouring immigration offenders tripled from 77 to 2012 to 233 in 2013, while that for employers of such offenders more than doubled from 41 to 97 over the same period.

The ICA said this was because it has taken over from the police all investigations pertaining to harbouring and employing immigration offenders, so as to centrally manage all immigration-related offences. It added that it has also embarked on an outreach programme to educate the public about the authority's role in border security, and to warn the public about immigration offences.

Other statistics included a 24.3 per cent spike in the number of contraband smuggling cases, to 99,700 in 2013 from 80,000 in 2012. Some items seized included birds' nest, loose tobacco, firecrackers, ninja stars and knuckle dusters.

"While smugglers of contraband items invent new ways of concealment, they also use old tricks to escape detection," the authority said. "The same method of concealment could also similarly be employed by terrorists to smuggle dangerous materials such as weapons or explosives into Singapore."

A new provision in the Immigration Act that kicked in on Dec 19, 2012 criminalising marriages of convenience also led to 124 people being convicted in 2013.

Another 160 were also found guilty of making false statements to obtain immigration clearance.