The head of Iran’s nuclear agency says his country has begun new operations at a heavy-water nuclear reactor.

The move intensifies pressure on Europe to find an effective way around U.S. sanctions, which block Tehran’s oil sales abroad.

Starting up the Arak heavy water reactor's secondary circuit on Monday does not violate Iran's 2015 nuclear deal with world powers.

But it does inch Tehran's program closer toward weapons-grade levels of nuclear material.

Heavy water helps cool reactors, producing plutonium as a byproduct that can potentially be used in nuclear weapons. Iran insists its nuclear program is for peaceful purposes.

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