UPDATE 3-Iran gives Shell/Repsol deadline on LNG project

* Deadline for Shell/Repsol expires on May 20

* Iran says it may talk with Chinese firms instead

(Adds no comment from Repsol spokeswoman)

TEHRAN, April 13 (Reuters) - Iran has given Royal Dutch Shell (RDSa.L: Quote, Profile, Research) and Repsol (REP.MC: Quote, Profile, Research) until May 20 to clarify their involvement in a major gas project in the Gulf, a senior oil official was quoted as saying on Monday.

"If subsequent to the expiry of the deadline these companies do not make clear their involvement in the Persian LNG project, talks will begin directly with Chinese (companies)," Seifollah Jashnsaz, managing director of the National Iranian Oil Company (NIOC), told the ISNA news agency.

"Presently not much remains to the end of this deadline," he said, without giving further detail.

A Repsol spokeswoman declined to comment on the INSA report.

The Persian LNG project concerns development of production and exports of liquefied natural gas from a part of the South Pars field in the Gulf. Repsol, Shell and NIOC signed an initial deal in 2002 to develop Phase 13 of South Pars.

Last year, Royal Dutch Shell delayed decisions on multi-billion dollar investments in Iranian LNG plans due to political tension. Repsol has large investments in Iran, which is also the world's fourth-largest oil producer.