Citing comments by Syrian nuclear chief Ibrahim Othman at a closed meeting Tuesday, the diplomats said the new structure appeared to be a missile control center or actual launching pad. The two - both from Western delegations to the IAEA - demanded anonymity for divulging details about what Othman told the International Atomic Energy Agency's 35-nation board.
Israel bombed the site in September 2007. While the Jewish state has not commented on the strike, Washington subsequently presented intelligence purporting to show that the target in a remote area of the Syrian desert was a nearly finished nuclear reactor built with North Korean help that would have been able to produce plutonium once completed.
Syria has denied secret nuclear activities but has blocked IAEA inspectors from visits beyond an initial inspection to the Al Kibar site.
Environmental samples from that trip have revealed traces of man-made uranium and graphite. But U.N. officials say it is too early to say whether the graphite - a common element in North Korean prototype reactors - had any nuclear applications.
Syria had previously said only that the site was military in nature and that it was being rebuilt. But the diplomats said comments by Othman suggested that the facility now in place of the bombed target was either a missile launching command center or a launching pad.
They quoted Othman as saying that, when IAEA Deputy Director Olli Heinonen visited the site in June, Heinonen was asked whether the Syrians should "put a missile in position" - apparently to demonstrate its present use - with the IAEA official saying no.
One of the diplomats said the briefing was told that the finding of 80 uranium particles in the environmental samples was "significant."
But Othman played down the laboratory results in comments outside the meeting - and denied outright that graphite was found. That denial contradicted comments from U.N. officials familiar with the Syria probe.
"There is no graphite at all," he told reporters. As for the uranium traces, "any analysis has errors," he said. "The smaller the amount the larger the (probability of) error."
One of the two diplomats also said that inside the briefing Othman announced that Syria would no longer accept evidence of apparent nuclear activity resulting from further findings from the samples taken by the agency.
That - and Damascus' continued refusal to allow other visits to the Al Kibar sites and other ones suspected of secret nuclear activity - could cripple the agency's investigative efforts.
Expanding on an IAEA report on the Syria probe circulated to board members earlier this week, agency officials told the meeting that Syria had apparently tried to secretly buy so-called "dual use" materials that can - but do not have to be - part of a nuclear program, said the diplomats.
Among the substances were high-grade graphite - used to control the speed of fission in some reactors - and barium sulfate, a nuclear shielding material. Syria claimed non-nuclear purposes for both substances, the diplomats said.
The briefers also said the uranium samples appeared inconsistent in shape, form and other details to back up Syrian claims that they originated from Israeli ordnance used to target the bombed site.
The Jewish state also has denied assertions by Damascus that the uranium came from Israeli depleted uranium bombs or warheads used to destroy the Al Kibar facility. Depleted uranium is used to harden munitions and increase penetrating power.
The agency officials at the briefing repeated earlier IAEA findings that satellite images and other evidence related to the bombed site indicated that the target and related buildings were similar in shape and other characteristics to a reactor.
A slide shown at the briefing said the overall findings of the probe so far suggested the finding was "not inconsistent" with a nuclear reactor scenario, said one of the diplomats.
Syria - and Iran's defiance of a U.N. Security Council demand it stop uranium enrichment - will be in focus at an IAEA board meeting starting next Monday that comes amid U.S. efforts to ease tensions with the two Mideast nations.
Syria's support for the militant organizations Hamas and Hezbollah has drawn the ire of Washington, which has also accused Syria of allowing foreign fighters to cross into Iraq. Syria, which staunchly opposed the 2003 invasion, insists it is doing all it can to safeguard its long, porous border.
Relations soured further when the Bush administration pulled the U.S. ambassador out of Syria in 2005 to protest Syria's suspected role in the assassination of former Lebanese Prime Minister Rafik Hariri. Damascus denied involvement in his death, but in the uproar that followed it was forced to withdraw its troops from Lebanon, ending a 29-year military presence.
Besides striving to improve relations with Damascus, the new U.S. administration has signaled it is ready to talk directly to Iran after a decades-long boycott over nuclear differences and other disputes.