European oil companies Royal Dutch Shell PLC and Repsol YPF SA have signed a preliminary agreement with Iran's state oil company on developing gas fields in the Persian Gulf, a Shell spokeswoman said. "We've signed an upstream service agreement as part of our work to assess the feasibility of the project" known as South Pars, Sarah Smallhorn said Monday. Smallhorn said the agreement, signed with National Iranian Oil Co. Saturday, follows a framework agreement the companies signed in 2004, but the final go-ahead for the project was not expected for another year - an apparent delay, since Shell said in February 2006 it then expected final approval in about a year. The United States and its allies have been pressuring banks and oil companies to pull out of oil and gas projects in Iran, due to Teheran's pursuit of nuclear technology for what it says are nonmilitary purposes. Some analysts believe oil companies may ultimately have to choose between doing business with the US or Iran.