Russian officials have rebuffed a new U.S. attempt to pry loose key secrets from their former biological weapons program, including a genetically altered strain of anthrax bacteria that Pentagon scientists are eager to study and that Russia had earlier promised to deliver, according to Sen. Richard G. Lugar (R-Ind.).

Government and security officials also balked at allowing a U.S. congressional delegation to visit one of Russia's four military-run biological research labs, which have remained closed to Americans despite a decade of cooperation between the two countries on securing stockpiles of nuclear, biological and chemical weapons.

The rejections came during a visit to Russia in late August by a delegation headed by Lugar, who is backing legislation to expand U.S.-Russian efforts to halt the spread of weapons of mass destruction. Despite progress in many areas -- including the destruction of hundreds of warheads, bombers and submarines -- the incidents underscore lingering "bureaucratic opposition" to the cooperation on terrorism pledged by President Vladimir Putin and President Bush at a summit last November in Texas, Lugar said.

"It shows that Putin is far ahead of much of Russia's bureaucracy on these matters," Lugar said Friday in a briefing to reporters.

But Lugar also warned against allowing the setbacks to undermine a 10-year-old U.S. commitment to help Russia destroy or secure its vast stockpiles of unconventional weapons -- a stockpile that Lugar describes as the United States' greatest security threat. Opposition in Congress to providing more assistance to Russia has delayed the opening of a U.S.-funded Russian facility built to incinerate nearly 2 million Soviet-era chemical weapons, potentially enough to destroy the world's population 20 times over, Lugar said.

Lugar said that at other stops on his trip, Russians worked closely with Americans to turn former weapons factories into research centers to cure diseases and reduce terrorism threats.

Lugar acknowledged he was unsuccessful during his visit in resolving a five-year dispute with Russia over a genetically modified strain of anthrax bacteria. The strain, developed by scientists at the Russian State Research Center for Applied Microbiology in the city of Obolensk, has been reported in scientific journals to resist many anthrax vaccines.

Eager to learn whether U.S. vaccines would work against the strain, the Defense Department in 1997 signed a contract with Russian researchers to acquire a sample. But Russia has refused to release the microbes, citing laws restricting the export of dangerous pathogens. Lugar pressed the issue Monday with senior Russian officials at Obolensk, and again three days later at a meeting in Moscow, but was given no firm commitment on the release of the strain.

Russia's refusal to honor the contract has been cited by some in Congress who oppose granting a permanent waiver that would free up millions of dollars in U.S. spending for nonproliferation projects in Russia. Last month, Bush signed a temporary waiver that restored funding only through Sept. 30.

Another sore point for the White House has been Russia's refusal to allow U.S. inspection of four biological research labs controlled by its Defense Ministry. While the U.S. government has provided millions of dollars to enhance security and retrain scientists at Russia's civilian-run bioweapons factories, the veil of secrecy surrounding military labs has fueled suspicions that Russia is continuing research on offensive weapons. Russia has said all research on offensive biological weapons has stopped.

One of the closed labs, the Center of Military-Technical Problems of Biological Defense at Yekaterinburg (formerly Sverdlovsk), was the site of an accidental anthrax release in 1979 that killed at least 68 people.

On Wednesday, Lugar's delegation traveled to another closed center, the Scientific Research Institute at Kirov, after receiving signals that a visit might finally be permitted. But despite an enthusiastic airport reception by Kirov's political leaders and news media, Lugar was refused entry to the military facility.

Defense Minister Sergei Ivanov had no explanation for the refusal during a meeting the following day.

Lugar said he warned Kirov officials they were jeopardizing their future by holding on to Soviet-era secrecy.

"They were interested in getting [Western] pharmaceutical companies to invest in these facilities," Lugar said. "But as I told them, it's a non-starter if investors can't even get inside the place."