A top US lawmaker pressed President Barack Obama on Thursday to impose new sanctions on Syria after photographs emerged suggesting its secret nuclear program was broader than previously revealed.

House Foreign Affairs Committee Chairwoman Ileana Ros-Lehtinen said she was "gravely concerned" after a Washington think tank published satellite photos that appeared to show Syrian efforts to conceal a second suspected nuclear site allegedly linked to a reactor bombed by Israel in 2007.

"Israel's destruction in 2007 of Syria's suspected uranium enrichment facility may have ended the immediate threat, but this new evidence indicates the regime has yet to abandon its nuclear goals," said Ros-Lehtinen.

The photos acquired by the Institute for Science and International Security (ISIS) indicate Syria tried to hide its nuclear activities after the air raid, and could lead the UN nuclear watchdog to step up demands for inspections.

Ros-Lehtinen said Obama "should increase pressure on the Syrian regime" with new sanctions as provided for under US law and "work with other responsible nations" to hammer out broader sanctions "without delay."

The lawmaker urged the International Atomic Energy Agency to insist on access to Syrian sites and said the watchdog should request a special inspection "which Syria is obligated by treaty to allow."

"It is dangerous that the administration continues to ignore Syria's threatening behavior and policies, and instead makes concessions to Damascus such as the recent appointment of a US ambassador," she said.

The photos show heightened activity and the pouring of a concrete foundation around the site near Marj as Sultan, outside Damascus, shortly after a May 2008 request for inspections by the International Atomic Energy Agency (IAEA).

"The facility's operational status is unknown. However, there is suspicion that Syria may have emptied the buildings prior to mid-2008 and taken steps to disguise previous activities at the site," the ISIS report said.

"Laying down a new foundation could be an attempt to defeat the environmental sampling that IAEA inspectors would likely carry out to see if uranium was present in the event of a visit to these suspect sites."

Citing the IAEA and the German newspaper Sueddeutsche Zeitung, ISIS said the Marj as Sultan facility was one of three sites that were "functionally related" to the bombed reactor at Dair Alzour, in the east of the country.

The full report is online at www.isis-online.org

Syria has snubbed a request by the UN nuclear watchdog for prompt access to a number of sites, and could come under heightened scrutiny at the Vienna-based IAEA's meeting next month, diplomats told AFP last week.

Damascus granted UN inspectors one-off access to Dair Alzour in June 2008 but no follow-up visits there or access to the other allegedly related sites.

On the basis of that one visit, the IAEA has already said the building bore some of the characteristics of a nuclear facility.

UN inspectors also detected "significant" traces of man-made uranium there, as yet unexplained by Damascus.

Syria has denied it is hindering the IAEA's work, and President Bashar al-Assad denied his country had pursued a nuclear program in an interview with the Wall Street Journal last month.