When Croatia becomes the 28th member of the European Union on Monday, it will shoulder a major task -- guarding the bloc's longest external land border in a region notorious for trafficking.

Croatia lies on the so-called Balkans route which stretches from Turkey across Greece, Macedonia and Serbia, and is used by criminal gangs to smuggle people, drugs or weapons.

Croatian authorities must monitor a border more than 1,300 kilometres (800 miles) long -- surpassing Finland's frontier with Russia -- which besides Serbia touches the non-EU states of Bosnia and Montenegro.

At the Karasovici crossing, nestled among pine-covered hills at Croatia's far south border with Montenegro, police official Stane Urlovic said: "Drugs and illegal immigrants are our main concern."

Only last year, some 6,500 would-be migrants were caught in Croatia on their way to Western Europe. They came mainly from Afghanistan, Syria, Turkey and Albania, and from the African nations of Somalia and Algeria.

Urlovic said illegal immigrants -- mostly men in their late 20s, but also couples with babies -- often try to sneak in only a few hundred meters (yards) away from the Karasovici crossing.

"Sometimes we catch the same people only a few hours after we have caught them for the first time and sent them back to Montenegro," she said.

Once an immigrant jumped out of a suitcase as an officer was opening it, she recalled. Another migrant was found rolled up in a carpet put under the children's feet in the backseat of a car.

Traffickers charge for a trip from Istanbul to an EU country between 5,000 to 6,000 euros ($6,700 to 8,000), police say.

EU border management agency FRONTEX said the illegal crossings in the region -- amounting to about 35,000 people -- rose by 33 percent in 2012 compared with the year before.

Also last year, around 600 kilos (1,322 pounds) of various drugs were seized at Croatian borders, and the smuggling of stolen vehicles is on the rise, police said.

To meet the border challenges, Croatia has spent tens of millions euros of European assistance funds to improve border infrastructure, modernise equipment and train police officers, said interior ministry official Gilio Toic Sintic.

Some 6,000 police officers are currently involved in the monitoring and control of the land borders with Montenegro, Serbia and Bosnia, but another 300 are needed, he said.

The 1,011 kilometre-long border with Bosnia is the most challenging not only for its length but also for its difficult terrain dotted with rivers and impassable mountains.

At a police station in Metkovic -- a southern Croatian town part of which lies in Bosnia -- mostly young officers monitor around the clock six 'suspicious points' on big screens.

"This is a suitable terrain for (illegal) migrants, they can hide easily and later 'get lost' in the town," said the head of the regional border police Mato Barisic.

Cameras cover these hot spots used by human or drug traffickers while Bosnian and Croatian officers have joint patrols in the town.

Barisic said Croatia's EU entry would not change much, although the country could now become a destination rather then just a transit point.

"We have been preparing for this for years," he said pointing to a nearby brand new Klek-Neum crossing on the Adriatic coast with Bosnia, where some eight million people, mostly tourists, pass every year.

But even with sophisticated equipment and new infrastructure, the human eye and long experience "are still what's most important in carrying out border controls while combatting trafficking," said Urlovic.

"A forged document can be recognised only by a good eye that can also recognise fear" in the person carrying it, she said.

Although becoming a EU member state on July 1, Croatia will not immediately join Europe's 26-nation passport free Schengen area. The newcomer aims to meet that goal by the end of 2015.