function obtenerPaises(prefijo){
	var pet = new ConstructorXMLHttpRequest();
	
	if(pet){

		//REF. COMBO CONTINENTE
		cont = document.getElementsByName("continente")[0];
		
		//REF. COMBO PAIS
		cmb = document.getElementsByName("pais")[0];
		cmb.innerHTML = '';
		cmb.disabled = true;
		
		param = "&continenteId="+cont.value;
		pet.open("POST",prefijo+"modules/buscar/_combos.php",true);
		pet.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
		pet.send("codOp=OP"+param);
		
		//CALLBAK
		pet.onreadystatechange = function(){
			if(pet.readyState == 4){
				var xml = pet.responseXML.documentElement;
				//Recorre el resultado
				if(xml.getElementsByTagName('pais').length > 0){
					for (i = 0; i < xml.getElementsByTagName('pais').length; i++){
						//Lee el nodo i-ésimo
						pais = xml.getElementsByTagName('pais')[i];
						id = pais.getElementsByTagName('id')[0].firstChild.data;
						nombre = pais.getElementsByTagName('nombre')[0].firstChild.data;
						//Crea un elemento OPTION
						option = document.createElement('option');
						option.value = id;
						option.innerHTML = nombre;
						cmb.appendChild(option);
					}
					cmb.disabled = false;
				}	
			}
		}
	}
}