"We have received a notification from the German police about the incident, but it does not say anything about who could be behind it," Ministry spokesman Krunoslav Borovec told reporters.

Commenting on claims by some media that the bomb was sent by an extremist organisation called the Croatian Phoenix, named after the so-called Bugojno Group which 40 years ago attempted an act of armed subversion against the Socialist regime, Borovec said this was speculation.

He added that the Ministry had offered all possible assistance to the German police in the investigation of the incident.

The package containing an M-75 hand grenade with a fuse and a safety pin and a threatening letter addressed to Croatian President Ivo Josipovic over his friendly relations with Serbian President Boris Tadic, arrived at the Embassy ahead of Josipovic's official visit to Germany this week.

Some Croatian media published the text of the threatening letter whose title contains a stylised Croatian coat-of-arms with a cross and the symbol HF, which the media believe stands for the Croatian Phoenix organisation.

"This little bomb is only a warning to the president of the UDBA-communist-antifascist-Chetnik alliance, comrade Josipovic, and his friend and ally Boris Tadic, the Chetnik duke, that March 31 this year is the 20th anniversary of the killing of Croatian policeman Josip Jovic at Plitvice Lakes. We have decided to avenge our knight by executing 300 Chetniks. No one can stops us in doing that," the letter reportedly reads.

Minister of Foreign Affairs and European Integration Gordan Jandrokovic said on Monday he was in close contact with the German police, but that there was still no information as to who was behind this "insane act".

Jandrokovic said that he had received assurances from his German counterpart Guido Westerwelle that the German authorities were guaranteeing security to the Croatian Embassy and to the Croatian President, who will arrive in Germany for a three-day visit on Wednesday.