(Adds Senate vote on Esfandiari, U.S. comment on sanctions)

By Matt Spetalnick

WASHINGTON, May 24 (Reuters) - President George W. Bush said on Thursday the United Nations should strengthen sanctions against Iran over its defiance of international pressure to curb its nuclear program.

"My view is that we need to strengthen our sanctions regime," Bush told a news conference a day after U.N. monitors said Tehran was flouting demands to halt its sensitive nuclear work.

Bush also called it "unacceptable" that Iranian authorities had recently detained several Iranian-Americans in Iran and said Washington had made its position clear to Tehran.

"I just spoke to Condoleezza Rice, and we will work with our European partners to develop further sanctions," Bush said when asked about the nuclear standoff. "And, of course, I will discuss this issue with (Russian President) Vladimir Putin, as well as (Chinese) President Hu Jintao."

"The first thing that these leaders have got to understand is that an Iran with a nuclear weapon would be incredibly destabilizing for the world. It's in their interests that we work collaboratively to continue to isolate that regime," he added.

Iranian President Mahmoud Ahmadinejad accused the West on Thursday of trying to stop Iran's atom program in order to reduce its influence in the world, and made clear it would not yield to the pressure.

Western powers suspect Iran is seeking to develop nuclear bombs, but Tehran says its activities are only aimed at generating electricity so that it can export more oil and gas.

The International Atomic Energy Agency said on Wednesday that Iran was making substantial advances in uranium enrichment in defiance of world demands, opening the way to harsher sanctions against Tehran.

The U.N. Security Council has imposed two rounds of sanctions on Iran since December.

The U.S. State Department said the United States would look at imposing a variety of sanctions, particularly those designed to hit Iran's economy, but declined to provide details.

A State Department official who asked not to be named said there was no talk of sanctions that would hit Iran's energy sector and that any fresh measures would be incremental.

"We're going to do this in gradual steps," he said.

In addition to the nuclear issue, the United States and Iran are at odds over Iraq, where Washington accuses Tehran of fueling the insurgency and of providing roadside bombs that have killed U.S. soldiers. Iran denies this.

Washington is also upset by Tehran's detention of four dual U.S. and Iranian citizens, including Washington-based scholar Haleh Esfandiari, who have been kept from leaving Iran.

The U.S. Senate on Thursday passed a resolution calling on the Iranian government to drop all charges against Esfandiari and immediately release her from prison.

(Additional reporting by Arshad Mohammed)

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