TIRANA (Reuters) - The United States urged its NATO ally Albania on Monday to start jailing bosses of organized crime if it hopes to make headway in its bid to join the European Union.

U.S. ambassador Donald Lu said fighting organized crime and corruption was “the biggest and most difficult challenge ahead” for the ex-communist Balkan country, which officially became an EU candidate in 2014 and hopes to begin accession talks soon.

“Until the big fish are arrested, prosecuted and go to jail, the cannabis will return, judges will be bribed, and government officials will be corrupted,” Lu told a gathering of judges and aspiring magistrates.

Citing U.S. reports, Lu described Albania as “a center of organized crime activity, which includes trafficking in drugs, weapons and prostitution”.

He said four major clans in Albania controlled 20 families involved in a wide range of criminal activities.

“Albania has a substantial black market for smuggled goods, primarily tobacco, jewelry, stolen cars and mobile phones. The country remains at significant risk for money laundering because of rampant corruption and a weak legal system,” Lu said.

Despite efforts to clean up the judiciary under EU pressure, a hit-and-run incident involving an armored car belonging to a powerful businessman and a police chief in the central town of Elbasan in September demonstrated again how some groups continue to see themselves as above the law.

After the incident, police stepped up their seizure of suspicious luxury cars, confiscating 17 armored cars.

Lu said he was troubled to see no major drug traffickers arrested or prosecuted in 2015 or 2016 when there was a sharp increase in the cultivation of cannabis in Albania.