BUCHAREST (Reuters) - Romanian oil firm Grup Servicii Petroliere (GSP) said on Tuesday one of its offshore drilling rigs in the Gulf came under attack by an Iranian military helicopter. "We were called by one of our employees at 9.15 a.m. local time (0615 GMT) who told us a military helicopter opened fire against the Orizont rig, and by 9.45 Iranian troops got on board," GSP spokesman Radu Petrescu told Reuters. "Since then, we haven't heard anything from them." The 13,000-tonne Orizont rig has 26 people on board and has been in Iranian waters since October last year. GSP said Dubai-based Oriental Oil Company accused it last week of illegally withdrawing another rig from Iranian waters. The Romanian firm says the contract regarding that rig, Fortuna, was canceled legally.