The International Atomic Energy Agency's 35-member board of governors convened at the agency's headquarters in Vienna for two days of deliberations on recent reports on the disputed nuclear dossiers of Damascus and Tehran.
The reports, circulated to the board last week, found that the IAEA was making little headway in either case.
Iran was said to be continuing to defy UN demands to cease uranium enrichment, a process used to make nuclear fuel and the fissile material for an atom bomb.
And IAEA Director General Mohamed ElBaradei also complained that Tehran was still refusing to answer multiple allegations of past nuclear weapons works.
The report on Syria was the first that the IAEA has issued after inspectors visited a suspect nuclear site in the remote Syrian desert in June.
The US claims that the site, Al-Kibar, had been a covert nuclear reactor close to completion, until it was razed to the ground by Israeli bombs in September 2007.
IAEA found that Al-Kibar did indeed appear to share some of the characteristics of a nuclear reactor and that traces of uranium had been found there.
Responding to the report's findings for the first time on Monday, the head of Syria's Atomic Energy Commission, Ibrahim Othman, ruled out any follow-up visits by IAEA experts.
"We will not allow another visit," he told reporters, insisting that Al-Kibar was a military site and Damascus therefore had no obligation to let UN inspectors in.
"No other country would allow any person to visit a restricted military site, just because he would like to see it," Othman said.
The Syrian official said uranium particles found by IAEA inspectors near the bombed site did not mean a reactor had been there.
But the IAEA insisted it would not let the matter rest there.
"No such nuclear material had so far been declared in Syria's inventory… In principle, that sort of nuclear material should not exist there. It's not usual to find man-made uranium in sand," an official close to the agency said.
On Iran, the IAEA said that after six years of intensive investigations, it was no closer to determining whether Iran's disputed nuclear drive was peaceful as Tehran claims.
"Regrettably, as a result of the lack of cooperation by Iran in connection with the alleged studies and other associated key remaining isuses of serious concern, the agency has not been able to make substantive progress on these issues," it said.
The IAEA also complained that Iran was refusing to suspend uranium enrichment as repeatedly demanded by the UN Security Council in New York.
Indeed, Iran had stockpiled approximately 630 kilogrammes (1,389 pounds) of low-enriched uranium or LEU, from the 3,800 centrifuges currently in operation, the IAEA said.
Estimates vary, but the UN watchdog has said it would need 1,700 kilogrammes of LEU to convert into high-enriched uranium (HEU) for use in an atom bomb.
