Bags of heroin are displayed during a news conference held by Bulgaria's national customs agency in Haskova, Bulgaria, July 7, 2011. (Xinhua/BTA)

SOFIA, May 21 (Xinhua) -- Bulgarian law enforcement authorities have seized 423 kg of heroin at Danube Bridge checkpoint near Vidin at the border with Romania, officials said here on Sunday.

Georgi Kostov, Director General of the National Customs Agency, said at a press conference that the illicit drug, split into 400 packets, was discovered on Saturday morning in a Bulgarian truck carrying terracotta tiles from Bulgaria to the Netherlands.

The purity of the heroin was extremely high, nearly 70 percent, Kostov said.

Authorities estimated the value of the seized heroin on the black market would exceed 36 million U.S. dollars, he added.

Mladen Marinov, Secretary General of the Ministry of Interior, said that the heroin was delivered to Bulgaria on Wednesday from Iran via Turkey by an Iranian truck.

After that, the heroin has been loaded on the Bulgarian truck, said Marinov, adding that the investigation is ongoing.

In April, Bulgarian authorities seized 214 kg of heroin hidden in a truck carrying cans of chicken breading spice from Iran to Poland.

Bulgaria, a key transit route for drugs smuggled from Asia to Europe, seized more than 500 kg of heroin in 2016, official data showed.