By Fredrik Dahl VIENNA (Reuters) - The U.N. nuclear watchdog signaled on Wednesday it would press Iran at talks in Tehran next month over concerns the Islamic Republic may have researched how to develop atomic bombs. Chief U.N. nuclear inspector Tero Varjoranta made the comments after what he and Iranian envoy Reza Najafi both described as a "productive" meeting held at the Vienna headquarters of the International Atomic Energy Agency (IAEA). They said they had reviewed progress in implementing the initial steps of a cooperation pact the two sides signed last month, under which Iran agreed to grant the U.N. agency access to two nuclear-related sites within three months. The agreement is meant to help allay international concerns about Iran's disputed nuclear program through a series of measures by Tehran to gradually open it up to greater scrutiny.