VIENNA (Reuters) - Radiation levels in a part of Chornobyl's exclusion zone where Ukraine has said Russian troops dug trenches in the highly contaminated soil are elevated but still well within the safe range, the U.N. nuclear watchdog's chief said on Thursday.

       International Atomic Energy Agency chief Rafael Grossi travelled to Chornobyl this week with IAEA staff to bring equipment and check radiation levels at the site which includes radioactive waste facilities near the now-defunct power plant that in 1986 suffered the world's worst nuclear disaster.

       Russian troops occupied Chornobyl soon after they invaded Ukraine in February and left in early April.

       "There was an increase (in radiation levels) but this increase is still significantly below the authorised levels for workers in an environment with this type of radiation," Grossi told a news conference.

       Grossi did not describe the excavation work near the highly contaminated so-called Red Forest in more detail, but said it was probably the work of Russian troops who occupied Chornobyl.

       IAEA staff on Wednesday measured a radiation dose level inside the excavations of 6.5 millisieverts per year, higher than the 1.6 millisieverts at nearby roads or a background level of 1 at IAEA headquarters in Vienna, but well below the limit of 20 that is considered safe for workers in areas exposed to radiation, according to a slide shown at the news conference.

       Asked if that meant it was safe to dig or spend time in those excavations, Grossi said: "I would say it was not a good thing (to dig them). I wouldn't recommend anybody to start excavating a place that is known to have been subject to high doses of radiation. There is a risk there."

       (Editing by Alexandra Hudson)