(Adds Cheney, statement from missing man's family)

By Sue Pleming

WASHINGTON, April 4 (Reuters) - The United States applauded Iran's decision to free 15 British sailors on Wednesday but said if Tehran wanted to change its relationship with Washington it would first have to suspend uranium enrichment.

In a rare approval of an Iranian decision, President George W. Bush welcomed the promise to free the 15 sailors while Vice President Dick Cheney said it was unfortunate the sailors were taken in the first place and he hoped there would be no "quid pro quo" for their release.

"Once people start taking hostages or kidnapping folks on the high seas and then are rewarded for it by getting some kind of political concession or some other thing of value, that would be unfortunate," Cheney told ABC News Radio.

In announcing he would free the 15 sailors, Iran's President Mahmoud Ahmadinejad said Tehran could also reconsider its relations with Washington if Bush's attitude changed.

"The behavior that needs to change is the Iranians, not the United States," State Department spokesman Tom Casey said.

Casey reiterated the United States would only deal directly with Iran if it gave up sensitive uranium enrichment work, which Washington believes is aimed at building a nuclear bomb. Iran says it is for peaceful power purposes.

"Iran can have a full set of discussions with the United States on any subject it wishes if it complies with the basic requirements of U.N. Security Council resolutions," Casey said.

Since the seizure of the sailors 13 days ago in what Iran says was its own waters and Britain says was Iraqi waters, the United States has softened its usual rhetoric against Iran, not wanting to make negotiations for their release even tougher.

U.S. Secretary of State Condoleezza Rice said on Monday if the circumstances were right, she would not rule out meeting Iran bilaterally, but with the usual caveat that Iran must give up enrichment.

MISSING AMERICAN

Previously, Rice has said she would meet with Iran only in a multilateral context, along with other nations dealing with Iran over its nuclear ambitions.

The United States is seeking Iranian help over a missing former FBI agent who disappeared in early March while on a business trip to Iran's Kish island, a free trade zone in the Gulf where Americans do not require visas.

Washington, which does not have diplomatic ties with Iran, sent an official inquiry via the Swiss Embassy in Tehran requesting information about the man, identified as Robert Levinson from Coral Springs, Florida.

Casey said Iran replied on Wednesday via the Swiss, who act as a go-between with Tehran.

"They asked us for additional information including our best understanding when he arrived in Kish island and other kinds of details about his travel," Casey said.

U.S. officials are concerned that Levinson, who is an expert on organized crime and the Russian Mafia in particular, may have been picked up by Iranian officials.

The timing of Levinson's disappearance is tricky for the United States, which is holding five Iranians in Iraq. Tehran has demanded their release and the fear is if the Iranians are holding Levinson, the cases could become linked.

"In our mind, there is no connection," said Casey.

State Department spokesman Sean McCormack said Iran's government had sought access to the five. "We will take that under consideration," said McCormack of the request.

Levinson's family issued a statement appealing for any information about their missing relative.

"We miss him and love him very much and are worried about him," said his statement.

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