Iran has amassed more than two tons of enriched uranium, the U.N. atomic agency said Monday in a report that heightened Western concerns about the country developing the ability to produce a nuclear weapon.

Two tons of uranium would be enough for two nuclear warheads, although Iran says it does not want weapons and is only pursuing civilian nuclear energy.

The U.S. and the four other permanent U.N. Security Council members — Russia, China, Britain and France — have tentatively backed a draft fourth set of U.N. sanctions against Iran over its refusal to stop enriching uranium.

Separately, the International Atomic Energy Agency — the U.N. nuclear watchdog — said Syria continues to stonewall agency reports to follow up on U.S. assertions that a facility destroyed three years ago by Israeli warplanes was a secretly built reactor meant to produce plutonium.