A threat by a militant group to use biological and chemical weapons if Israel invades the Gaza Strip is probably a bluff, an expert here said on Monday. The Al-Aksa Martyrs' Brigades (the militant wing of Palestinian Authority Chairman Mahmoud Abbas' Fatah faction) said it would surprise Israel with new kinds of weapons if Israel invaded the Gaza Strip. "With the help of Allah, we are pleased to say that we succeeded in developing over 20 different types of biological and chemical weapons...after a three-year effort" the group said in a leaflet distributed on Sunday. "We say to Olmert and [Defense Minister Amir] Peretz: Your threats of invasion do not frighten us. We will surprise you with new weapons you have not faced until now," they said. "As soon as an [Israeli] soldier sets foot on Gaza land, we will respond with a new weapon."The group said that it would declare "open warfare without limits" if Israel invaded the Gaza Strip. But Israeli counter-terrorism expert Dr. Ely Karmon described the statements as "wishful thinking." They already indicated that they had chemical weapons in 2003, he said. It's "psychological warfare," said Karmon. Hamas tried to put chemicals in explosive belts worn by suicide bombers and it didn't work, he said. Israeli military sources would not say whether army intelligence believed the claims to be true or not. If and when the Israeli army enters the Gaza Strip, the troops will be prepared, the sources said.