BELGRADE (Reuters) - Serbian and German police arrested 30 people in a suspected drug smuggling ring and seized narcotics worth around 20 million euros ($22 million) on Tuesday, Serbia's interior minister said.

Police seized 303 kg (670 lb) of cocaine, 106 kg of hashish and 96 kg of marijuana as well as weapons and money, after investigations lasting 13 months.

Suspects were arrested in Croatia, Germany and Serbia on Tuesday, with 21 others arrested in Spain, Belgium and Hungary between October 2015 and March 2016, Germany's federal police said in a statement. Three further suspects remain at large.

German police said the drug smuggling ring was suspected of selling drugs across Europe from bases in Belgrade and Essen, Germany, and maintaining illegal "indoor plantations" of marijuana at numerous sites in both countries.

Serbia, which lies on key drug-smuggling routes between Central Asia and the wealthy markets of Europe, has stepped up efforts to combat organized crime and corruption to fulfill conditions for joining the European Union.

It was the first time the two countries had carried out a joint police investigation, German prosecutor Anette Milk said. “The cooperation worked very well,” she told Reuters.

"Serbia will continue to cooperate with Germany in all cases related with organized crime and corruption," interior minister Nebojsa Stefanovic said.

Since the ouster of former president Slobodan Milosevic in 2000, cooperation between Serbian and foreign police has improved dramatically. In 2014, a group of Serbian cocaine traffickers were arrested in an operation involving the CIA.

In separate raids last week, Serbian police arrested 68 people around the country on charges ranging from violent crime to drug trafficking.

(Reporting by Aleksandar Vasovic; Additional reporting by Andrea Shalal in Berlin; Editing by Robin Pomeroy and Hugh Lawson)