WASHINGTON, July 9 (Reuters) - Iran has made only "modest" progress in its nuclear program because of U.N. sanctions, said a senior U.S. official on Wednesday, but he warned of high costs to Tehran if it continued sensitive nuclear work.

"While Iran seeks to create the perception of advancement in its nuclear program, real progress has been more modest," U.S. Under Secretary of State for political affairs, William Burns, said in prepared testimony to the House of Representatives foreign affairs committee, adding pressure would intensify on Tehran if continued its current course.

Burns' testimony came amid increased tensions with Iran, which on Wednesday test-fired nine missiles and warned the United States and Israel it was ready to retaliate for any attack over its disputed nuclear projects. (Reporting by Sue Pleming; Editing by Kristin Roberts)

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