(Recasts lead, adds details from Treasury briefing, comment by
White House official)

By Corbett B. Daly

WASHINGTON (Reuters) - The U.S. Treasury Department
Tuesday sought to further tighten curbs against Iran's
nuclear ambitions and its support for terror groups by naming
Iranian-controlled companies and forbidding dealing with them.

In addition to naming the 21 companies, Treasury cited
leaders of Iran's Islamic Revolutionary Guard Corps-Qods Force
(IRGC-QF) for providing financial support to the Taliban and
other groups.

"Iran is the primary founder of Hizbollah and has long been
recognized as the most active state sponsor of terrorism,"
Treasury said, adding that it was targeting IRGC-QF by naming
four of its senior officers.

Among the four, it said General Hossein Musavi and Colonel
Hasan Mortezavi were providing financial and material support
to the Taliban. The Treasury Department had previously
designated the IRGC-QF in October 2007 as an organization that
supported terror groups but went a step further on Tuesday by
naming its leaders.

The department also listed 21 companies or entities in six
countries -- engaged in businesses as far-flung as banking,
mining and engineering -- that it said were
Iranian-controlled.

U.S. law prohibits Americans from doing business with the
government of Iran. The Obama administration said it was naming
the firms so that people can more easily avoid the risk of
dealing with them and accidentally violating sanctions against
trade with Iran.

"As its isolation from the international financial and
commercial systems increases, the government of Iran will
continue efforts to evade sanctions, including using
government-owned entities around the world that are not easily
identifiable as Iranian to facilitate transactions in support
of their illicit activities," said Stuart Levey, Treasury's
undersecretary for terrorism and financial intelligence.

At a briefing, Levey described Iran as "by far the most
prolific funder of terrorism in the world."

The 21 companies are in Japan, Germany, Italy, Belarus,
Luxembourg and Iran and are not easily recognized from their
names as being Iranian-controlled. For example, one of them is
a Tokyo-based company called Ascotec Japan K.K. and another is
a Hamburg, Germany-based company called West Sun Trade GmbH.

Treasury also added two Iranian organizations -- the
Iranian Committee for the Reconstruction of Lebanon and the
Imam Khomeini Relief Committee (IKRC) Lebanon branch -- to its
terrorism blacklist.

The action, under an executive order, freezes any assets
they may have under U.S. jurisdiction and prohibits Americans
and U.S. firms from engaging in any transactions with them.

"Today's designations expose Iran's use of its state
apparatus-including the Islamic Revolutionary Guard Corps-Qods
Force-and state-run social service organizations to support
terrorism under the guise of providing reconstruction and
economic development assistance or social services," the
Treasury said.

At the White House on Tuesday, spokesman Robert Gibbs was
dismissive of a call from Iranian President Mahmoud Ahmadinejad
for a televised debate with U.S. President Barack Obama to
discuss who has the best solutions for world problems.

"We have always said we would be willing to sit down and
discuss Iran's illicit nuclear program if Iran is serious about
doing that," Gibbs said, adding "To date, that seriousness has
not been there."

(Additional reporting by Matt Spetalnick and Glenn
Somerville; Editing by Bill Trott)