Suspicions that China is facilitating illegal North Korean arms exports have gained new credence as authorities investigate a plane carrying weapons from Pyongyang that was detained during a refueling stop in Thailand.

The Russian-made Ilyushin-76, with a crew of four Kazakhs and one man carrying a passport from Belarus, was impounded Friday carrying 35 tons of weapons, reportedly including unassembled Taepodong-2 missile parts. The destination of the plane was not confirmed, but specialists said Iran was likely.

Larry A. Niksch, a specialist in Asian affairs at the Congressional Research Service who monitors North Korea's proliferation activities, said the Bangkok seizure raises serious questions about China's role.

"Two-thirds of the flight path of that plane was over Chinese territory," he said. "It should have raised Chinese suspicions."

The Obama administration brought up concerns about North Korean use of Chinese airspace for arms exports this summer - shortly after the adoption of a U.N. Security Council resolution banning such transfers - but has yet to receive a meaningful response, U.S. officials said.

"North Korean proliferation by air is an important matter for us, and [Philip] Goldberg brought it up during his meetings in July," said one official, referring to an Asia trip by the State Department envoy for the implementation of Resolution 1874. The official spoke on the condition of anonymity because he was discussing private diplomatic communications.

The resolution, which China supported, lists detailed procedures on how to deal with suspicious vessels and illegal cargo on the high seas, but it is somewhat vague when it comes to air cargo.

In most cases, regardless of the destination of a flight originating in North Korea, it would have to refuel in China or at least fly over its territory, Mr. Niksch said.

China's state-run Xinhua News Agency quoted officials in Beijing in July as saying that inspections of air cargo should be carried out only if there is specific evidence of wrongdoing.

"China has been faithfully implementing relevant U.N. resolutions," Wang Baodong, a spokesman for the Chinese Embassy in Washington, said Wednesday. "As to whether the North Korean plane violated U.N. resolutions, it's up to the U.N. Security Council to make a judgment."

Victor D. Cha, senior adviser at the Center for Strategic and International Studies and a senior official in the George W. Bush administration, said Chinese officials see "it as too big a step for them" to inspect planes coming from North Korea. He said China's goal is "to balance just enough pressure to bring the North back to [nuclear] talks but not so much as to collapse them."

"It is one of the hardest lifts on the counterproliferation side with China. If they close off airspace, that would make a huge difference in counterproliferation efforts. It's easier to stop a boat than a plane," he said.

Mr. Cha, who was senior director for Asian affairs at the National Security Council under Mr. Bush, said the Bush administration "always raised this with China in the context" of the so-called Proliferation Security Initiative, which is aimed at preventing dangerous weapons and materials from falling into the hands of rogue states or terrorists.

U.S. nonproliferation policy in recent years has focused on seaborne cargo, but analysts say North Korea prefers air traffic for transfers of weapons, technology and scientists because it is harder to track. The incident in Thailand marks the first time air cargo from the North has been intercepted. Cargoes from several ships have been intercepted in recent years.

The Obama administration has been trying to persuade North Korea to return to six-nation talks on its nuclear program, but Pyongyang has been resisting. The State Department said Wednesday that U.S. special envoy Stephen Bosworth delivered a letter from President Obama to North Korean leader Kim Jong-il during Mr. Bosworth's visit to North Korea last week, but declined to share details.

Many questions remain about the plane detained in Bangkok, which was searched after a tip from U.S. intelligence, Thai officials said. One question is why the plane did not refuel in China. Another is whether Beijing was aware that the Americans were tracking the flight, Mr. Niksch said.

Despite initial reports that the weapons were destined for another country in the region - with Myanmar being the chief suspect - specialists now view Iran as a more likely candidate.

The five crew members have refused to talk to the police but have said in published interviews that their destination was the United Arab Emirates, which has been a transit point for clandestine North Korean arms shipments to Iran in the past.

As recently as July, United Arab Emirates authorities uncovered an arms shipment from North Korea at the Dubai port. The weapons found in Bangkok appear to be similar to those in Dubai, specialists and Thai officials said.

One Thai official familiar with the investigation was quoted Wednesday by Reuters news agency saying that "some of the components found are believed to be parts of unassembled Taepodong-2 missiles."

Pyongyang's long-range Taepodong-2 is a product of joint efforts with Iran, coinciding with Tehran's development of the Shahab-5 and Shahab-6 missiles. Iran on Wednesday test-fired another missile, the Sajjil-2, a two-stage, surface-to-surface missile that has a potential range of 1,200 miles.

"We need more information on the types of weapons found in Bangkok, but the preliminary information indicates that some of the weaponry is of the type that Iran regularly supplies to Hezbollah," Mr. Niksch said in reference to the Lebanese militant group.

"We know that Iran has rearmed Hezbollah substantially since the 2006 war with Israel," he said. "The discovery in Dubai should leave no doubt that North Korea has been involved in this."