“None of the missiles tested by Iran is capable of carrying a nuclear warhead,” Foreign Ministry spokesman Ramin Mehmanparast said.

British Foreign Secretary William Hague told Parliament on Wednesday that Iran was testing missiles capable of delivering nuclear payload.

Iran is carrying out a 10-day military exercise in a show of strength it hopes will warn Israel and the United States against any attack.

As part of the exercise, which started on Monday, Iran test-fired 14 surface-to-surface missiles on Tuesday with a maximum range of 2,000 km.

Iran has “been carrying out covert ballistic missiles tests and rocket launches, including testing missiles capable of delivering a nuclear payload,” Hague said.

Britain believes Tehran has conducted at least three secret tests of medium-range ballistic missiles since October, amid an apparent escalation of its nuclear program and scrutiny from the International Atomic Energy Agency.

Hague also said Britain was concerned over Tehran’s decision to increase its capacity to enrich uranium to a higher level at the Fordo site near the holy city of Qom in central Iran.

“It has announced that it intends to triple its capacity to produce 20 percent enriched uranium. These are enrichment levels far greater that is needed for peaceful nuclear energy,” Hague said.

Low-enriched uranium — at around 3.5 percent — can be used to fuel a reactor to generate electricity, while uranium enriched to around 90 percent purity can be used to develop a nuclear warhead.

Enriching to 90 percent can be done much more easily from material enriched to 20 percent purity than from low-enriched material.

Some Western officials claim Iran is accelerating its attempts to produce a nuclear weapon, partly because leaders have been shaken by protests against authoritarian regimes across the Middle East.

“We will maintain and continue to increase pressure on Iran to negotiate an agreement on their nuclear program,” Hague said.