Here is the reply I received from BPELSG, which is the exact same text shown on their FAQ document:
If the applicant is applying for licensure as a civil engineer, and the experience offered has taken place in the United States, all references must be licensed engineers authorized to practice civil engineering, or federal employees (see Question 26). If a civil engineer applicant has had qualifying work experience outside of the United States, the references for that experience must be from persons authorized to practice civil engineering in accordance with the laws of the country in which the experience took place. If such references are not licensed civil engineers, they must provide information, which indicates by what authority they are authorized to practice civil engineering.
If the applicant is applying for licensure as an electrical or mechanical engineer, all references must be from engineers authorized to practice in the discipline in which the applicant is seeking licensure, by virtue of licensure or by exemption. Since licensed civil engineers are authorized to perform supplementary electrical and mechanical engineering work, such persons may serve as references if they meet other requirements. See Question 26 for information on satisfactory references.
If the applicant is applying for licensure in any of the remaining branches of professional engineering, all references should be from engineers, scientists, or technical persons who meet the satisfactory reference requirements of Question 26. Since these other branches are not practice regulated, anyone is authorized to practice in those branches, and thus anyone can serve as a reference. However, the reference must have sufficient knowledge of the applicant's engineering specialty to be able to make knowledgeable comments about the applicant's performance on the job.
I believe it means that in my case the international registered engineers will be acceptable. @CAPLS can you kindly chime in?