William Hague told the House of Commons on Wednesday that there had been secret experiments with missiles and rocket launchers.
Iran is conducting 10 days of war games in an apparent show of strength to the West and on Tuesday fired 14 missiles in public tests.
Britain believes Tehran has conducted at least three secret tests of medium-range ballistic missiles since October.
Iran and the West remain in dispute over its nuclear program. The U.S. and its allies insist it is aimed at developing atomic weapons – a charge Iran rejects.