BEIJING – Iran’s hard-line president, Mahmoud Ahmadinejad, arrived in China last night for a summit of Asian states and Russia that Washington fears is forming a new anti-Western alliance.

Mr. Ahmadinejad will seek support for his country’s nuclear program, fueling American concern that Iran is being protected by its growing friendship with Russia and China, who both sit on the U.N. Security Council.

He also is believed to be pushing to join the Shanghai Cooperation Organization, which is holding its annual summit in the city today and tomorrow.

The SCO, under the leadership of China and Russia, is playing an ever-greater role in the jostling for power in Central Asia. The dictatorial nature of some of its membership, which also includes the former Soviet republics of Uzbekistan, Kazakhstan, Kyrgyzstan, and Tajikistan, has provoked descriptions of it as an anti-American alliance of despots.

Both China and Russia deny this, and China’s vice foreign minister, Li Hui, this week ruled out early accession by Iran. But Vladimir Putin, Russia’s president, said in a newspaper article: “The SCO is not a closed and exclusive club whose lines of demarcation have been clearly drawn.”

Defense Secretary Rumsfeld has expressed concern about the summit and the presence of Mr.Ahmadinejad as an official observer.

“It strikes me as strange that one would want to bring into an organization that says it’s against terrorism … one of the leading terrorist nations in the world,” he said.

President Hu of China and Mr. Putin are likely to assure the Iranians that they will oppose the use of strong U.N. sanctions or force against Iran if it refuses to stop enriching uranium that could be used for nuclear weapons. Mr. Ahmadinejad, who in speeches at home has called repeatedly for Israel’s destruction, will be given a rare opportunity to appear on an international stage.

The importance of the event to China, which is trying to extend its regional clout and exploit America’s growing unpopularity, was underlined by an extraordinary security operation in the financial hub of 17 million people. Shanghai has been instrumental in the explosive economic growth of the world’s largest country.

Extra police patrolled main roads, while two underground train lines leading to the meeting site in Pudong in the east of the city were expected to close ahead of the leaders’ arrival. A force of 60,000 security officers has been deployed including armed police, soldiers, and bomb-sniffing dogs. Dissidents have been placed under house arrest and many workers given the rest of the week off.

The SCO comprises the four former Soviet republics plus Russia and China, with Pakistan, India, Iran, and Mongolia holding observer status.

Its stated goal is economic cooperation and joint action against terrorism, particularly Islamist groups that have been active in all six countries. But while this provided common cause with America in the aftermath of the attacks of September 11, 2001, all have been accused of subordinating human rights to sometimes overblown claims of combating extremism.