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(CNN) -- In the face of economic sanctions and international condemnation, Iran remains defiant over its nuclear energy program.

It has threatened to close the Strait of Hormuz, a major passageway for much of the world's oil, in retaliation for any embargo. And it has vowed to punish the United States and Israel,accusing them of being connected to the recent assassinations of some Iranian nuclear scientists.

U.S. and Israeli officials have denied having anything to do with the mysterious killings, but they're not backing down on their hard-line stance.

"As long as (the Iranians) shirk their responsibilities, this pressure will not relent," U.S. President Barack Obama said Tuesday night in his State of the Union speech. "Let there be no doubt: America is determined to prevent Iran from getting a nuclear weapon, and I will take no options off the table to achieve that goal. But a peaceful resolution of this issue is still possible, and far better."

Tehran insists it's pursuing nuclear energy for civilian purposes, not for military use. The U.S. and its allies aren't buying it.