Although Iran and North Korea have received far more attention, Syria is belatedly becoming a prime focus of international concern over nuclear proliferation.

On December 1, the Institute for Science and International Security released satellite photos of suspected Syrian nuclear sites linked to the covert Al-Kibar nuclear reactor that Israeli warplanes bombed in 2007, shortly before it could begin operations. The CIA later confirmed that the site contained a North Korean–designed nuclear reactor that would have been capable of producing plutonium for nuclear weapons.

Syria blocked inspectors from the International Atomic Energy Agency (IAEA) from investigating the suspected nuclear facility until after it had cleaned up the bombed site to remove incriminating evidence. Under strong international pressure, it reluctantly permitted IAEA inspectors a brief visit in 2008 but since then has stonewalled IAEA efforts to investigate its nuclear program.

On Thursday, IAEA Director General Yukiya Amano said that he sent a letter to Syrian Foreign Minister Walid al-Muallem formally requesting access to suspected nuclear sites. If Syria continues to block the IAEA investigation, its failure to comply with its nonproliferation commitments can be referred to the United Nations Security Council for further action. Similar nuclear defiance by Iran led to an escalating series of U.N. sanctions.

Iran is suspected of financing the covert Syrian reactor in an end run to acquire plutonium for nuclear weapons without risking the discovery of additional nuclear facilities on its own territory. Syria, which has little oil, does not have the economic resources or technological infrastructure to independently build an expensive nuclear weapons program. The tyrannical Syrian and Iranian regimes are close allies that have developed strong ties with North Korea, an outlaw nuclear power that has provided important military and nuclear assistance to both.

In February, Western officials leaked the fact that before Israel destroyed the Syrian reactor in September 2007, North Korea had delivered 45 tons of un-enriched uranium “yellow cake” to Syria and subsequently moved it to Iran via Turkey after the Israeli strike.

Despite U.N. sanctions, North Korea has continued to provide both Iran and Syria with missiles, components, and technology, and the recently revealed North Korean uranium enrichment plant at Yongbyon poses an additional proliferation risk. The Yongbyon facility appears to have a design similar to Iran’s uranium enrichment facility at Natanz. North Korea may have obtained Iranian help on uranium enrichment in exchange for its extensive assistance for Iran’s ballistic missile force, which is largely based on North Korean missile technology. Or there may have been a nuclear quid pro quo in the form of a transfer of its nuclear technology or plutonium to Iran.

Whatever the arrangement is, it is clear that Tehran and Pyongyang have secretively undertaken extensive nuclear cooperation. According to a senior North Korean defector: “The nuclear power and missile research institutes in the North and Iran are effectively one body. North Korean nuclear and missile scientists are in Iran and Iranian scientists are working in the North. They share everything.”

While Iran and North Korea have been forced by sanctions to pay a growing price for their nuclear defiance, Syria has so far escaped any consequences. This should be rectified as soon as possible. Polite letters from the IAEA will have little effect unless they are strongly backed by an international coalition determined to punish Damascus if it continues to drag its feet on its nonproliferation obligations. This requires firm American leadership.

The Obama Administration could start by abandoning its diplomatic wooing of Syria and taking a harder line on Syria’s nuclear violations. A bipartisan group of eight congressional leaders wrote a December 2 letter to President Obama urging him to take stronger action against Syria’s nuclear program. The signers included Representatives Jeff Fortenberry (R–NE), Edward Markey (D–MA), Ed Royce (R–CA), and Brad Sherman (D–CA) and Senators John Ensign (R–NV), Kirsten Gillibrand (D–NY), Jon Kyl (R–AZ), and Joseph Lieberman (I-D–CT).

If the Obama Administration fails to provide the strong leadership needed to ensure more effective action at the IAEA and the U.N. Security Council, then Syria, a junior partner in the nuclear axis of evil, will continue to advance its illicit nuclear program with impunity.