BAGHDAD, Nov 6 (Reuters) - Iran has helped to "rein in" Shi'ite militias blamed for fuelling Iraq's sectarian violence, Iraq's foreign minister said on Tuesday, calling it a positive development ahead of a planned new round of security talks.

Hoshiyar Zebari's comments in a telephone interview with Reuters follow a noticeable softening in the language used by U.S. officials to describe Iran's role in Iraq in the past week.

Washington has repeatedly accused Tehran of supplying rockets, mortars and roadside bombs to Shi'ite militias in Iraq as well as offering training on how to use them, a charge Tehran denies.

But in recent days U.S. officials have spoken of a drop in the number of armour-piercing bombs that Washington says Iran has supplied to militias, and noted Shi'ite cleric Moqtada al-Sadr's call on his Mehdi Army to observe a six-month ceasefire.

"Iran has been instrumental in reining in the militias and the Mehdi Army by using its influence," Zebari said. "Part of the security improvement was their control of the militias. We see this as a positive development."

While U.S. officials have hinted at a link in the sharp drop in violence in Iraq to what they see as Iran's sway over the militias, Zebari's comments appeared to be the first by a senior Iraqi government official on the issue.

He hailed an announcement by the U.S. military earlier in the day that it planned to free nine Iranians in its custody, saying it was a confidence-building measure that would encourage more productive talks between Iran, Iraq and the United States on improving security in Iraq.

"We have tried very hard with the American military and the (U.S.) embassy to release them. We are very pleased now that a decision has been made," he said.

"We believe it will help enhance confidence in dialogue between Iran, Iraq and the United States on Iraq security and will make (for) more useful and substantive talks."

He said a security sub-committee set up earlier this year that includes representatives of Iraq, the United States and Iran would meet again soon. Both Washington and Tehran have indicated their willingness to attend the talks.

Zebari said the Iraqi government would continue to lobby for the release of Iranians still being held by U.S. forces, a sensitive political issue that has raised tensions between Iraq and Iran, which has demanded they be freed. (Editing by Sami Aboudi)