CZECH investigators have discovered explosives at the Palestinian embassy complex in Prague where a possibly booby-trapped safe killed the ambassador on January 1, police say.

Police discovered 12 illegal weapons following the explosion at the embassy that killed Ambassador Jamal al-Jamal, but this is the first time that authorities said explosives were also found in the new complex that includes the embassy and the ambassador's residence.

It remains unclear what caused the safe to explode but the ambassador's death is being investigated as a case of negligence.

Al-Jamal had only started his posting in October.

Police spokesman Tomas Hulan said they have sent the explosives for testing at Prague's Institute of Criminology.

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

The ministry said on Thursday it would not immediately comment on the explosive.

Police said on Thursday they received a letter from the Palestinian authorities saying the weapons were given to them as gifts by the officials of the former communist Czechoslovakia.

Czechoslovakia was part of the Soviet bloc that had warm ties with the Palestinian Liberation Organisation.

According to police, the Palestinians said the weapons were never used and always kept in a safe.

Police said experts are trying to determine whether the weapons were used in any criminal activity in the past.

The ballistic testing might take weeks, police said.

Originally published as More explosives at Czech Palestine mission