* Environmental groups must argue case against projects
* No deadline to file details complaints to regulator (Adds comments from BSH president, context)
FRANKFURT/DUESSELDORF, Jan 21 (Reuters) - Construction work on the Nord Stream 2 gas pipeline could be stopped for an indefinite period in German waters but opponents still need to give detailed reasons to justify the halt, Germany’s federal maritime regulator BSH said on Thursday.
Two environmental groups this week filed complaints with BSH against a move to expand the period during which construction work could theoretically take place, effectively preventing further work on the pipeline for the time being.