In this answer, I will assume that we are talking only about surveilling the foreign terrorists, not trying to infiltrate their group. If this is the case, there are likely to be at least two dynamics that will make it harder to effectively conduct surveillance.
First, there is the fact that your agents are likely to stick out like the proverbial sore thumb. If the terrorists are operating in a closed ethnic community, it is likely that people in the community all know one another. They are likely to spot outsiders and to be suspicious of them. This will make it very difficult to conduct surveillance without being detected.
Second, the actions of your agents are likely to cause some degree of discontent on the part of the community. This could cause problems. The community might not truly realize that there are terrorists among them. They might feel that they are the victims of ethnic profiling. This will make them resent the agents and they will probably not cooperate with those agents in any way. They might even try to make it harder for the agents to do their jobs. In a democracy, they might launch protests or otherwise try to push back publicly against the surveillance.
Thus, conducting surveillance is likely to be difficult in the circumstances described here.