KIGALI (REUTER) - Aid officials on Wednesday discussed ways to tighten security in Rwanda after gunmen killed four U.N. human rights monitors in the latest in a wave of attacks. There have been eight attacks on expatriates in Rwanda in the last three weeks. Marie Van Der Elst, a spokeswoman for the U.N. human rights office in Kigali, said the bodies of British and Cambodian monitors killed in the Tuesday ambush were flown back to the capital from the southwest town of Cyangugu late on Tuesday. Two other monitors - both Rwandans - were killed with them on a visit in a marked U.N. vehicle to Karengera commune in the southeast, bordering on Zaire. Their Rwandan driver was wounded in the stomach and died later in hospital during surgery. Security fears forced monitors in the northwest provinces of Ruhengeri and Gisenyi back to the capital following the killing in cold blood of three Spanish aid workers on January 18. Claude Dusaidi, an adviser to Vice-President and Defence Minister Major-General Paul Kagame, said he was disgusted "at the actions of these thugs". U.N. officials said they were investigating the exact circumstances of the ambush and few details were available. "One thing not mentioned in this story is the people being targeted in this attack were our own (government) ministers who were on a sensibilisation visit to the area," Dusaidi said. He gave no details. Government ministers tour the country trying to reconcile the Hutu majority and Tutsi minority in the wake of the return from Zaire and Tanzania from November to December last year of more than one million Hutu refugees. U.N. Secretary-General Kofi Annan said he was "shocked and dismayed" at the killings, which U.N. sources said were particularly brutal. There are 139 U.N. human rights officers in Rwanda. The U.N. human rights office reported on Friday Hutu death squads were killing Tutsi genocide survivors and witnesses to the slaughter, apparently to stop them denouncing people for the 1994 bloodbath. U.N. humanitarian sources in Geneva said the U.N. human rights office was likely to come under pressure to pull out its 139 observers from the tiny Central African country to avoid further attacks. U.N. High Commissioner for Human Rights Jose Ayala Lasso strongly condemned the killings.