The United States and five other world powers pressed Iran on Thursday to grant U.N. inspectors access to alleged nuclear research sites inside the country, amid reports that Iranian officials are moving to clean up a key military installation outside Tehran.

The request came as Western officials confirmed that satellite photos in recent weeks have shown earth-moving equipment and other construction gear arriving at the Parchin military base, a facility about 20 miles southeast of the capital that has become the focus of a U.N. investigation into alleged Iranian research on nuclear warheads.

U.S. and European officials are concerned that Iran is trying to sanitize the site to remove any traces of nuclear research.

A statement Thursday by the countries involved in nuclear talks with Iran expressed concern about its leaders’ failure to open Parchin and other suspect sites to inspectors from the International Atomic Energy Agency. The six countries — the United States, Russia, China, Britain, France and Germany — noted that Iran had blocked U.N. delegations from entering the base in January and February.

“We urge Iran to fulfill its undertaking to grant access to Parchin,” the statement said.

Iranian officials said Tuesday that IAEA inspectors would be allowed to visit the site, but they later said that no visit would be permitted until all outstanding disputes between Iran and the U.N. agency had been resolved.

The dispute comes as the six world powers are preparing for the first nuclear talks with Iran in more than a year.

Secretary of State Hillary Rodham Clinton told reporters in Washington that it was essential that Iran show “openness and transparency” about its past nuclear activities if the talks were to be successful.

“We continue to believe we have space for diplomacy,” Clinton said. She called for Iran to be “prepared to have the kind of serious and sincere discussion we have been looking for, for several years.”

In a rare positive remark about the United States, Iran’s supreme leader lauded President Obama on Thursday for saying that he was trying to avert war with the Islamic republic.

“Two days ago, we heard the president of America say: ‘We are not thinking of war with Iran.’ This is good. Very good. These are wise words. This is an exit from illusion,” said Ayatollah Ali Khamenei, according to state television.

But he accused the U.S. president of “still harboring illusions” that sanctions would force Iran to give up uranium enrichment.

The timing of the remarks, in Khamenei’s first speech after parliamentary elections last Friday, appeared to reflect a boost in self-confidence among Iranian leaders after what officials said was a high turnout.

“If there is any deal to be made with the West, the timing is now,” said one analyst, who spoke on the condition of anonymity. “Khamenei in his speech is saying that he will respond to positive remarks by the U.S.”

Parchin is an Iranian military testing facility that U.S. officials believe was used a decade ago to test explosive triggers of the kind used to detonate nuclear warheads. Iran allowed a limited visit to the site by an IAEA team in 2005, but no evidence of nuclear activity was found. Iran denies that it has ever sought to develop nuclear weapons.

If Iran is indeed moving to clean up the site — first reported late Wednesday by the Associated Press — it would not be the first time it used such tactics. In 2004, when the IAEA sought to visit a military-run physics laboratory, Iranian authorities razed the building.

Warrick reported from Washington.