Czech police say investigators have discovered another explosive at the Palestinian embassy complex in Prague where a possibly booby-trapped safe killed Ambassador Jamal al-Jamal on New Year's Day.

Spokesman Tomas Hulan confirmed the discovery on Thursday, adding the unidentified explosive was sent for testing to Prague's Institute of Criminology. He declined to give details.

Police previously found 12 illegal weapons in a search following the January 1 explosion, prompting the Czech Foreign Ministry to accuse the Palestinians of breaching international obligations.

It remains unclear what caused the safe to explode but the ambassador's death is being investigated as a case of negligence. However, the discovery of the explosive and weapons are considered illegal arms possessions.

The Palestinians have officially apologised for the incidents after the Czech Foreign Ministry demanded an explanation, accusing the Palestinians of breaching international obligations. 

According to the police, they received a letter from Palestinian authorities claiming the weapons were given to them as gifts by officials of the former communist country of Czechoslovakia. Czechoslovakia was part of the Soviet bloc that had warm ties with the Palestinian Liberation Organisation.

The Palestinians said the weapons were never used and always kept in a safe.

Police said experts are trying to determine - through ballistic testing - whether the weapons were used in any criminal activity.

Source: AP